Using a merge tag in Send To Email + other text

I'd like to set Sent To Email: to list-subscribe=user={UsersEmail:1}@domain.com where {UsersEmail:1} is a merge tag referring to a field called UsersEmail containing a user-supplied email.

I can already set it to {UsersEmail:1}, but it doesn't validate if I add anything else. It's either type in an email address or use a merge tag by itself, but nothing else.

Is there a way to disable the validation for this field so it'll go through?

Or is there a way to take the user-supplied email address in one field (like joe@joeworks.com), then take that, perform a modification to it (like list-subscribe-user=joe@joeworks.com@domain.com) and store it in a different hidden field so that would be available to in the notification panel?

[I could've sworn I submitted this question yesterday but I can't find any trace of it today. I guess I didn't hit submit or something]