as-product-required.php

add_filter( 'wpas_get_custom_fields', 'wpas_make_product_required' );
/**
 * Make the products field required
 *
 * @param array $custom_fields Registered custom fields
 *
 * @return array
 */
function wpas_make_product_required( $custom_fields ) {

	if ( isset( $custom_fields['product'] ) ) {
		$custom_fields['product']['args']['required'] = true;
	}

	return $custom_fields;

}