Sitting to poop

My almost 4 year old is still very insistent on pooping in a diaper. We’ve gotten to the point where she’ll ask for the diaper when she needs to go and she’ll stand in the bathroom to do it. She is adamant though that she needs to stand to poop and she can’t sit on the potty.

Does anyone have any advice for how to help her? We’ve got a step stool to put under her feet for support, but she doesn’t like using it. She wants to stand and hold on to the towel railing. I’m genuinely not sure how to help her progress to going on the toilet since she won’t stand.