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:
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