forminator-submission-id-sheet.php

<?php

add_filter( 'forminator_addon_googlesheet_form_submitted_data', function( $submitted_data ) {
	global $wpdb;
	$entry_id = $wpdb->get_var( "SELECT `AUTO_INCREMENT` FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = '{$wpdb->dbname}' AND TABLE_NAME = '{$wpdb->prefix}frmt_form_entry';" );
	if( $entry_id ){
		$submitted_data['hidden-1'] = ( $entry_id - 1 );
	}
	return $submitted_data;
});