On a 4000T model I plugged in a Logitech wired keyboard into the side USB socket and it worked a treat.
Most keys have no effect, but the following keys have the function described:
Up or down arrow = List of codes and associated TV channels on left hand side of screen.
Pause Break = Pauses programme, then hit Enter to resume.
Home = Home screen for TV Guide, Recordings, iPlayer, Netflix etc., then Escape to return to live TV
Escape = Return to live TV
$ = On Demand. Left-pointing arrow to see Recordings.
Numeric keys: to enter code, e.g., 704 for Radio 4
The keyboard is especially useful when searching for a film title on NetFlix.
Please reply if you find any other keys with a useful effect.
I note that the manual makes no mention of “keyboard” after a keyword search. I think the feature is there to aid Humax developers. Of course, if features are undocumented then manufacturers can change them at will.