Hi,
We can't add drop down list to select a domain to join.
Perhaps we have learned how to add drop down list to select a domain Ou to join according to the link: https://scriptimus.wordpress.com/2013/02/11/mdt-2012-domainous-list/
This achieves the goal by edit customsettings.ini. But this is a solution that already offers us in the official Microsoft documentation. And there no message about joins different domain. For more details:
Maybe you can have other ways to join a different domain, but I haven't used it myself and don't know if it's possible. Here is the link:
https://www.reddit.com/r/MDT/comments/yid57f/mdt_and_multiple_domains_how_to_handle_single/
--please don't forget to upvote and Accept as answer if the reply is helpful--