How to let users define the URLs for establishing OAuth connection

Looking to setup a system that allows folks to connect to their server via oauth connection. Each server has a different subdomain/domain for the authorization flow so I would like them to enter their base URL in their profile and then use that to construct the URLs used in the authentication flow. I was hoping to use variables in the API plugin to achieve this but does not seem possible.
Anyone have a good workaround to achieve this type of setup?