Post custom field default value

How do I set a default date for a date custom field I’m using in GMW form?

I have tried the following to no avail….

function gmw_modify_form_values( $gmw ) {
	
	if ( 4 !== absint( $gmw['ID'] ) ) {
		return $gmw;
	}
	
	$startDate = date("Y-m-d");
	
	$gmw['form_values']['start-date'] = $startDate;

	return $gmw;
}
add_filter( 'gmw_default_form_values', 'gmw_modify_form_values', 50 );