Big props here to Amazon Prime Video for allowing you to open an Amazon app you've already logged into on a device with a camera and scan a QR code on your television to log in, rather than requiring you to type a password using arrow keys to select from a virtual keyboard.
If you have an iPhone, you can use the Apple TV remote on the phone which will show a text input you can type on (on any form text input). This has the added bonus of being able to access your keychain for automatic password filling
I have enabled the tv remote on control center - I’m not sure if it’s there by default. You should be able to toggle it in settings