Using API plugin to let users connect to their own environment

I am starting to play around with the api plugin, have it working for several use cases where I am going to the same endpoint.

I have another use case though, where I would like end users to log into their particular instance of a product. All of the access/token URLs are based on their particular subdomain, so I cannot just put in a standard URL for any of the oauth flows, is there a way to set that up as a variable so users can enter their subdomain, then enter their login credentials?

Thanks in advance!