J.P.
My daughter had trouble with this until she started school this year. She would prefer to do it in her pants and said she was afraid of the toilet. PLEASE do not yell at her or show frustration it only makes it worse. Just keep putting her on the toilet or a potty seat. Try giving her a toy or book to look at while she is sitting. I also keep a step stool in the bathroom for my daughter to put her feet on, this way her legs won’t hurt from hanging.
I think it is a control thing, once she feels she is in control of sitting on the toilet she will use it. You can also try bribes, a piece of candy or keep a few dollar store toys and let her pick one.
Just remember she will not do this forever, she will grow out of it. Please have patience with her.