Can't access my MGM+ account from flex.

I have an account with MGM+. TV's native menu, my android mobile phone, my wife's iphone, and my firestick all have apps that allow me to watch it, and I can watch it on my PC through a web browser.

When I try to access MGM+ through the flex, it doesn't seem to have an option for entering existing user credentials.

The only option it gives me is to subscribe (for an extra monthly charge on my bill), but since I'm already a subscriber, that would be a waste of money.

Is there a way to use my existing MGM+ account on the flex, or is the flex limited to using a bespoke version of the app that's deliberately designed to squeeze more money out of customers? If that's the case, it strikes me as a rather predatory practice. 

MGM+ is (so far, at least) the only service where I've encountered this issue. I use my existing accounts with other streaming services just fine on the flex, despite having signed up for them individually, outside of the xfinity/flex.

What's different about MGM+ that it disallows using an existing subscription account, OR how do I go about entering the credentials for the MGM+ subscription that I'm already paying for?