Tv.Apple.com pop up window does not allow data entry
I have a mid 2011iMac that I want to use to stream movies and TV programmes from Apple TV plus. It has a big beautiful screen that I am loathed to discard.
However, it cannot run Catalina OS - it is stuck on High Sierra OS - and so the Apple TV app is not downloadable. It has no HDMI input ports so connecting an Apple TV device is not an option.
Other threads suggest I should just log onto tv.Apple.com and watch movies from there (I have a subscription). However the pop up box asking me to Sign in with your Apple ID’ has the Apple ID field greyed out and sat behind the black text. I therefore can’t even identify myself let alone enter a password.
Any suggestions?
iMac 27″, macOS 10.13