To pre-populate a single form select list from an external source, choose the select field and click on 'Edit'.
Under the 'Choose Options Binder' menu, click on 'Bean Shell Form Binder' in the Options Binder select field.
Under 'Configure Bean Shell Form Binder', enter your Bean shell script to populate the select fields. For each row that you add to the FormRowSet, be sure to put the 'FormUtil.PROPERTY_VALUE' and 'FormUtil.PROPERTY_LABEL' fields in the FormRow that will be added to the FormRowSet.
Of course, you would want to populate the values for PROPERTY_LABEL and PROPERTY_VALUE from your external datasource. For more information on how to connect to a database source, see the example code within 'Pre-populate Form Fields with Data from External Source' or this example on the Joget Blog.