There not really a fool proof way… I know if I put a towel on the floor or take litter box away, I have one rabbit that will go pee there. You could try something a folded towel on the floor. Buns just read to pee on soft things like this.

A towel in the carrier is a good idea anyway, so they don’t slip around so much or in case they do pee in there.

There is a temptation to pee on soft things, so maybe make a sacrificial pee box before leaving? Put fluffy stuff in a box, put your rabbit in it, and see if it makes them pee? Personally, I know my Wick pees whenever I put him someplace “new”, so he wants to mark somewhere, so that’s a full-proof way for us — though I have this suspicion that rabbits always have a reserve pee in them for marking, because I can feel really confident about Wick not peeing because he just did, but bam, it happens again, haha.

I have that suspicion also.

Another suggestion: clean out & refresh litterbox. The often want to jump in and use it right away.

Mine will immediately jump into a freshly cleaned litter box… just so they can be the first to use it.