Referral working with restricted coupons

functions.php

<?php

add_filter('woocommerce_apply_with_individual_use_coupon', 'filter_apply_with_individual_use_coupon', 10, 4);
function filter_apply_with_individual_use_coupon($apply, $the_coupon, $applied_coupon, $applied_coupons) {
    // Check if the coupon code starts with 'ref'
    if (strpos($the_coupon->get_code(), 'ref') === 0) {
        $apply = true;
    }
    return $apply;
}