custom url template

Thanks for the report, you’ve actually hit a real bug, and I’ve just released version 1.5.6 that fixes it.
In 1.5.6, the check accepts any of {domain}, {sld}, or {tld}, so any of these URL patterns will now work:
https://yourbilling.com/order/?domain={domain}
https://yourbilling.com/cart.php?a=add&sld={sld}&tld={tld}
https://yourbilling.com/register/{domain}/

What you need to do after updating

1. Update the plugin to 1.5.6.
2. Go to WH4U Domains → Settings → Shopping Cart Redirect.
3. Make sure your custom URL contains at least one of {domain}, {sld}, or {tld} , written exactly like that, with curly braces, lowercase, no spaces.
4. Make sure the URL starts with https:// (plain http:// is rejected for security).
5. Save, then test by clicking Register on an available domain on your frontend, you should now be redirected to your billing system with the domain pre-filled instead of seeing the plugin’s form.

If you’d like to share your custom URL (with the placeholders, not a real customer’s domain), I’m happy to double-check the format for you.

Example:

https://example.com/order-domain/{domain}/

Click on mybusiness.com → goes to:

https://example.com/order-domain/mybusiness.com/

Your page (a custom page template, a [shortcode], or a Gutenberg block) reads the URL segment and shows a tailored “You’re about to register mybusiness.com” page.

Thanks again for taking the time to report this, it was a real bug and your message is what got it fixed.

i will mark this issue as solved.
markup options is schedule for the next release next week.