Me being a Dutchman, it will have to be in the dyke. (This is silly. We can't have that! Oops, sorry, Monty Python reference on the loose in the wrong thread. #13, the larch. Nudge nudge, wink wink.)
But anyroads, the thumb goes onto the end of the fingerboard on the fretless, the same on the fretted but there it will often move to the neck pickup.