Source code for https://formidable-masterminds.com/use-toggle-to-copy-fields-in-form-sections/

toggle-trigger.js

<script>
jQuery(document).ready(function($) {
    "use strict";

    // populate primary contact name and email from registration credentials
	$("#field_devdir_copy_name_and_email").on("change", function() {
		if ( $(this).prop('checked') ) {
			bindCurrent();
		} else {
			unbindCurrent();
		}
	});

	var bindCurrent = function() {
		// First copy values
		$("#field_devdir_primary_contact_first").val($("#field_devdir_first_name").val()).attr("readonly",true); 	//first name
		$("#field_devdir_primary_contact_last").val($("#field_devdir_last_name").val()).attr("readonly",true);		// last name
		$("#field_devdir_primary_contact_email").val($("#field_devdir_email").val()).attr("readonly",true);	    	// email

		/* Then bind fields */
		/* first name */
		$("#field_devdir_first_name").keyup(function() {
			$("#field_devdir_primary_contact_first").val($(this).val());
		});
		
		/* last name */
		$("#field_devdir_last_name").keyup(function() {
			$("#field_devdir_primary_contact_last").val($(this).val());
		});
		
		/* email */
		$("#field_devdir_email").keyup(function() {
			$("#field_devdir_primary_contact_email").val($(this).val());
		});
	};

	var unbindCurrent = function() {
		/* unbind fields */
		$("#field_devdir_first_name").unbind("keyup");
		$("#field_devdir_last_name").unbind("keyup");
		$("#field_devdir_email").unbind("keyup");
		
		/* clear readonly and disabled attributes */
		$("#field_devdir_primary_contact_first").removeAttr("readonly");
		$("#field_devdir_primary_contact_last").removeAttr("readonly");
		$("#field_devdir_primary_contact_email").removeAttr("readonly");
		
		/* clear fields of copied values */
		$("#field_devdir_primary_contact_first").val("");
		$("#field_devdir_primary_contact_last").val("");
		$("#field_devdir_primary_contact_email").val("");
		
		/* set focus to primary contact first name */
		$("#field_devdir_primary_contact_first").focus();

	};
});
</script>