Require Downloadable fields – Downloadable product template

add_filter( 'wcvendors_pro_product_form_download_files_path', 'make_required' );

add_filter( 'wcvendors_pro_product_form_product_attributes_path', 'make_required' );

add_filter( 'wcv_product_dowlnoad_limit', 'make_required' );

add_filter( 'wcv_product_download_expiry', 'make_required' );

function make_required( $field ){

	$field['custom_attributes'] = array( 'required' => '' );

	return $field;

}