Plugin Development - elementselect question

Dear all, 

I am developing a plugin which i would like to use the "elementselect" type to make my list dynamic as the LoadFormOptions So I  have used the property: options_ajax to implement

But when I configure the plugin in UI it said NoClassFound

json of the plugin

StFunciton is the interface I have created

public class StInterface extends DefaultApplicationPlugin

public class StCreateAccount extends StInterface implements StFunction

The error is ClassNotFoundException

Do anyone have some ideas about this?

Or give me some advice of how to use the options_ajax property of the elementselect field?

PS I am using Joget V3

Regards,

Jianan