Can I use this to attack someone I don't like/ competitor's website?
NO, strictly NO. Period. This service is for legal testing of YOUR OWN websites and servers only. You must prove ownership of any website you request an attack on.
I'm just curious how to prove ownership?
If my domain is anonymous, thats a no go
I can upload files but that doesn't prove i own it, just means i have access.
If you have a VPS do you really own anything to test?
Does this stress test only apply to port 80?