Why Does My Cat Pee Outside The Litter Box?
These Cats Are Marking Their Territory
Be glad in fact that they are doing it only in the litter box. Some things can help, such as making sure you have plenty of boxes if you have multiple cats (the standard advice is one for each cat plus one more) but even that may not help if your cats REALLY don't want to share. Feline calming pheromones like Feliway may help if the cat is feeling a bit stressed and is territorial for that reason.
What should I do?
As long as your cat is able to jump into the box, all you have to do is give her an open container with litter in the bottom in place of her other box. If you prefer an enclosed pan, or your cat can't jump that high or is too nervous to do so, you can cut an opening in one side for her to crawl through. Just make sure it's as high up as is practical for the cat.
You can move it a bit at a time until it is snapped in place and the box is fully covered. Does your cat use this sort of litter pan, or have you found something else that works? Let us know in the comments below!