I would assume a straight bit, of the relevant size would be best (the slot would probably be 19mm internal, but the width of the track itself? I have never done this, so I am now going to step back and let a real carpenter answer, because destroying your bench would be more responsibility than I could cope with.
I have mounted T tracks by using a track saw to cut the sides of the slot to very-slightly-deeper than required, then using the router to take out the material between. I find the track saw gives a better cut than the router, but I don't see why you couldn't cut out the slot one side at a time using say a 12mm or 16mm router bit. I probably wouldn't want to do a full depth cut, but take it down gradually in 2 or 3 passes.