Will the different servers always be the same API endpoint, so that for instance SEP API is hosted on Server A B and C and you want to chose A to get the token for and work with?
It's a bit of a hack but you could have the base url be just "https://" then make the variable be with the path, in this case [a] (rename to URL etc)
This does assume you've already generated the Components using the correct Base URL in the previous screen, got your RequestContent/ResponseContent working then modified this later.
[a] will then become a variable in your WF scope you can edit.