How do you convince your parents that writing on fanfiction.net or fictionpress.com isn’t bad?

My parents grounded me from my precious writing because they think it’s a waste of time. if i don’t get some of my stories posted im going to explode. (internally)
PLEASE HELP!!!!!!!!
also the category i chose was the closest thing i could find that pretained to my question.
DO NOT answer if you don’t know what fanfiction.net [...]