I have something to add to the good advises you received: do a little pile with the katkor, so your cat can scratch in.

When mine doesn’t want to pee in, I put his classic litter in the box, a litter bag over it, and the katkor over the bag, but that only works if your cat doesn’t pierce the litter bag.

Keep the pee in the fridge if you have to wait before bringing it to the vet.

I get this while getting log in. I have to click on any link and go back on the page to charge it.