Setting up a separate User for Apple TV +

Have just subscribed to Apple TV+ which I stream via an Apple 4K device. Am trying to create my wife as an additional user on her smart TV but no joy. She can see and use my profile but cannot create her own profile. I have created an Apple ID for her but when I try and create an additional User using her Apple account details it tells me that she needs to have an iTunes account. Have also tried to set up family sharing but this goes nowhere. She gets the invitation but cannot respond to it as she is not on an Apple device. If I try and respond on my Apple phone it goes nowhere. Can't help feeling that I'm missing something simple here!