If you are talking about the Humax iPlayer app in on-demand, you just need to follow the on-screen intructions.
I.e. Go to bbc.com/account/tv, register and key in the 8-character code displayed.
Despite the video playing in the background stating you only need to do this once, if you sign out from the Humax app, you will have to repeat the process.
At the moment it is optional but this will change.
Presumably, this is something that is rolling out. I am able to access iPlayer without entering any code – and that has been the case ever since I had the 4000T. A couple of times I was asked to indicate (YES/NO) that I had a TV licence, which seemed superfluous – as you could just answer in the affirmative whatever your licence status might be. R-