This Gist shows how to use JS to get and modify form data.

custom.js

jQuery( document ).ready( function() {
    var runCustomJS = Marionette.Object.extend( {

        initialize: function() {
            this.listenTo( Backbone.Radio.channel( 'form' ), 'render:view', this.editFieldData );
        },

        /*
         * In this function you can do things like: 
         * modify the form data
         * modify field data
         * etc. 
         */
        editFieldData: function( view ) {
           //Get form data. 
           var formModel = view.model;
           var fieldCollection = formModel.get( 'fields' );
           
           //To update a specific field by key. 
           var emailField = fieldCollection.findWhere( { key: 'email' } );
           emailField.set( 'value', 'someValue' );
           //Tell the field to re-render.
           emailField.trigger( 'reRender' );
           
           //Update a field by ID.
           var htmlField = fieldCollection.get( 12 );
           htmlField.set( 'value', 'someValue' );
           //Tell the field to re-render.
           htmlField.trigger( 'reRender' );
           
        }

    });

    new runCustomJS();
});