I'm not familiar with Com100 and integrations. But you'd have to extent that side.
Typical what SSO does is when you get to place X it refers you to Place Y. If you're not logged in you get the login at that side whatever that is (in your case Forms auth). If you where logged in that just gives place X a note who you are.
Have a look at the documentation for more info:
docs.xperience.io/.../configuring-single-sign-on