There are 4 key web socket methods that you are required to pay attention to:

public void onOpen(Session session)
- This method will be invoked when a WebSocket connection is opened. It send a specified text message to client side.

public void onMessage(String message, Session session) 
- This method will be invoked when a message is received on the WebSocket. It will send a response back to the client to indicate that the server has received the message.

public void onClose(Session session) 
- This method will be invoked when WebSocket connection is closed. It will log an information message indicating that the connection is closed.

public void onError(Session session, Throwable throwable)
- This method will be invoked when an error occurs in the WebSocket communication. It will log an error message.

package org.joget;

import java.io.IOException;
import java.util.Map;

import javax.websocket.Session;

import org.joget.apps.app.service.AppPluginUtil;
import org.joget.apps.app.service.AppUtil;
import org.joget.apps.form.lib.Radio;
import org.joget.apps.form.model.FormBuilderPalette;
import org.joget.apps.form.model.FormData;
import org.joget.apps.form.model.FormRow;
import org.joget.apps.form.model.FormRowSet;
import org.joget.apps.form.service.FormUtil;
import org.joget.commons.util.LogUtil;
import org.joget.plugin.base.PluginWebSocket;
import org.joget.workflow.model.service.WorkflowUserManager;

public class WebSocketPlugin extends Radio implements PluginWebSocket {

    private final static String MESSAGE_PATH = "message/form/WebSocketPlugin";

    @Override
    public String getName() {
        return "WebSocketPlugin";
    }

    @Override
    public String getVersion() {
        return "5.0.0";
    }

    @Override
    public String getClassName() {
        return getClass().getName();
    }

    @Override
    public String getLabel() {
        //support i18n
        return AppPluginUtil.getMessage("org.joget.WebSocketPlugin.pluginLabel", getClassName(), MESSAGE_PATH);
    }

    @Override
    public String getDescription() {
        //support i18n
        return AppPluginUtil.getMessage("org.joget.WebSocketPlugin.pluginDesc", getClassName(), MESSAGE_PATH);
    }

    @Override
    public String getPropertyOptions() {
        return AppUtil.readPluginResource(getClass().getName(), "/properties/form/" + getName() + ".json", null, true, MESSAGE_PATH);

    }

    @Override
    public FormRowSet formatData(FormData formData) {

        FormRowSet rowSet = null;

        // get value
        String id = getPropertyString(FormUtil.PROPERTY_ID);
        if (id != null) {
            String value = FormUtil.getElementPropertyValue(this, formData);
            if (value != null) {
                // set value into Properties and FormRowSet object
                FormRow result = new FormRow();
                result.setProperty(id, value);
                rowSet = new FormRowSet();
                rowSet.add(result);
            }
        }
        return rowSet;
    }

    @Override
    public String renderTemplate(FormData formData, Map dataModel) {
        String template = "webSocketPlugin.ftl";

        WorkflowUserManager wum = (WorkflowUserManager) AppUtil.getApplicationContext().getBean("workflowUserManager");
        String username = wum.getCurrentUsername();
        dataModel.put("username", username);

        String html = FormUtil.generateElementHtml(this, formData, template, dataModel);
        return html;
    }

    @Override
    public String getFormBuilderTemplate() {
        return "<label class='label'>" + getLabel() + "</label>";
    }

    @Override
    public String getFormBuilderCategory() {
        return FormBuilderPalette.CATEGORY_CUSTOM;
    }

    @Override
    public void onOpen(Session session) {
        try {
            session.getBasicRemote().sendText("Connection established");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onMessage(String message, Session session) {
        try {
            session.getBasicRemote().sendText("Server received: " + message);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onClose(Session session) {
        LogUtil.info(getClassName(), "Webscoket connection closed");
    }
    
    @Override
    public void onError(Session session, Throwable throwable) {
        LogUtil.error(getClassName(), throwable, "");
    }

    public boolean isEnabled() {
        if ("true".equalsIgnoreCase(getPropertyString("enableWebsocket"))) {
            return true;
        } else {
            return false;
        }
    }

}

The endpoint is specified at this line:

const ws = new WebSocket(((window.location.protocol === "https:") ? "wss://" : "ws://") + window.location.host + "${request.contextPath}/web/socket/plugin/org.joget.WebSocketPlugin");

The following 4 functions are event handlers that handles different WebSocket Events :

ws.onopen = function(event)
ws.onmessage = function(event)
ws.onclose = function(event)
ws.onError = function(event)

You may make changes to any of the FTL code as it suits.

<div class="form-cell" ${elementMetaData!}>
    <label class="label">
        ${element.properties.label} <span class="form-cell-validator">${decoration}</span>
        <#if error??>
            <span class="form-error-message">${error}</span>
        </#if>
    </label>
    <#if (element.properties.enableWebsocket! == 'true')>
        <input id="isEnabled" name="isEnabled" type="hidden" />
    </#if>

    <#if (element.properties.readonly! == 'true' && element.properties.readonlyLabel! == 'true') >
        <div class="form-cell-value"><span>${value!?html}</span></div>
        <input id="${elementParamName!}" name="${elementParamName!}" type="hidden" value="${value!?html}" />
    <#else>    
        <input type="text" id="messageInput" placeholder="Enter message">
        <button id="sendButton">Send</button>
        <button id="closeButton">Close</button><br>
        <div id='output'></div>
    </#if>

    <script>
    $(document).ready(function() {
        $("#sendButton").off("click");
        $("#closeButton").off("click");

        if ($("#isEnabled").length > 0) {

            const ws = new WebSocket(((window.location.protocol === "https:") ? "wss://" : "ws://") + window.location.host + "${request.contextPath}/web/socket/plugin/org.joget.WebSocketPlugin");

            ws.onopen = function(event) {
                console.log(event);
                $("#output").append('Connection opened with timeStamp: ' + event.timeStamp + '<br/>');
   
                $("#sendButton").on("click", function(){
                    const message = $("#messageInput").val();

                    //send message to endpoint
                    ws.send(message);  
                    $("#output").append("${username} send " + message + '<br/>'); 
                    $("#messageInput").val("");
                    return false;
                });

                $("#closeButton").on("click", function(){
                    if (ws.readyState === WebSocket.OPEN) {
                        //close the endpoint connection
                        ws.close();
                    }
                    $("#sendButton").hide();
                    $("#closeButton").hide();
                    return false;       
                });
            }; 

            ws.onmessage = function(event) {
                $("#output").append(event.data + '<br/>');
            }; 

            ws.onclose = function(event) {
                $("#output").append('Connection closed with timeStamp: ' + event.timeStamp + '<br/>');
                $("#output").append('WebSocket closed<br/>');
            }; 

            ws.onError = function(event) {
                $("#output").append("Error: " + event.data + '<br/>');
            };  
        
        } else {
            $("#output").html('WebSocket is not enable.<br>');
        }
    });
    </script>
    <div style="clear:both;"></div>
</div>