custom-generator.php

<?php

add_filter(
	'jet-form-builder/forms/options-generators',
	'add_custom_jfb_generator'
);

function add_custom_jfb_generator( array $generators ) {
	$generators[] = new class extends \Jet_Form_Builder\Generators\Base {

		public function get_id() {
			return 'custom_generator_id';
		}

		public function get_name() {
			return __( 'Custom generator name', 'jet-form-builder' );
		}

		public function generate( $args ) {
			// Get value from option "FIELD NAME"
			$field_name = $args['generator_field'] ?? false;

			// Single option must have "value" & "label" property
			$row = array(
				'value' => 1,
				'label' => 'First Value'
			);

			return array( $row );
		}
	};

	return $generators;
}