How to add a custom domain registrar to Domain Mapping and integrate it with Pro Sites?

I’m using Pro Sites and I’m planning to use Domain Mapping to integrate it with a different domain registrar than eNom and WHCMS. I would like to know if the API of this other domain registrar must be linked to the Domain Mapping plugin or if it has to be linked to Pro Sites.

I don’t want users to register a domain on the dashboard. I want the user to register the domain when they select their package on Pro Sites.

I want the user to register on Pro Sites a subdomain and also a domain on the frontend registration page and that domain that they entered must also be linked to the subdomain they entered via domain mapping.