Since you’re using SSO, the login HTML isn’t part of the Discourse site - it is part of the SSO provider.
If you want to obtain the HTML for the login page, you need to make a normal request to https://discourse.gomomentum.org/, without an API key, and follow the HTTP redirect chain. (although, I’m not sure why you want the login HTML, that seems like an unusual requirement…)
If you just want a way to test the API, make a request for https://discourse.gomomentum.org/about.json. You just need the API key and username, no need for any SSO information.