So this is how whmcs do it....
The backend looks like https://prnt.sc/kM8kAJUkXeNp where we would put the domain that can be used for sub-domain hosting and customers would see it something like https://prnt.sc/jOBwpqa2KsVY
I'm trying to setup namecheap for domain registration. When testing i'm getting "Unable to dispatch provision request due to missing data" other than that i don't have any more information to go on or any idea what the cause of the issue is?