How To Get Stray Kitten To Use Litter Box?

My kitten is 10 months old and she won’t use the litter box. I’ve done everything I can think of to train her, but nothing works. If she’s not in the box for a while, she’ll pee on my shirt or couch or wherever without any prompting from me. She is completely house trained and goes outside every time we leave the room (she even poos outside). Is there anything else you guys could suggest?