When I’ve had to do this in the past, I was fortunate that my vet gave me a tad more guidance than it sounds like you’ve gotten.

Put that litter in an empty and clean box, and leave it and the cat in a closed room, ideally a bathroom. Check every few hours to see if the cats peed.

When I’ve had to do this I was able to leave the cat in the bathroom overnight; when things were more time critical in the vet gave me the option to just have them take a urine sample directly from the bladder in the office.

It’s Question from the show Justice League Unlimited, probably with Lex Luthor under the Vecna face, so probably the episode “Question Authority”.