@Anonymous User
I'd highly recommend asking that question on the Apple Discussions website to get a definite answer to your question, as its my understanding that target display mode was only designed to display another Mac on your iMac display, and whilst it's easy enough to output from your iMac to another HDMI display using a cheapo adapter, persuading your iMac to allow it to use its own display for another (non-Mac) HDMI device isn't so simple and would require something more expensive. But as its not something I have experience of myself I'd definitely try and get an answer on Apple Discussions, there's plenty of far more knowledgeable Mac users on there (most of the users are US-based and won't be familiar with NOW TV boxes, just say you want to hook up a Roku box, it's the same thing).
https://discussions.apple.com/welcome
However there should be no need to go to that trouble as you should be able to stream easily enough from the browser/app. Are you able to stream from other services well, and have you tried removing the app are re-installing?
