Is it possible to have a frame linked to the sign up page of another site? Are there a security issues which makes this a bad idea and/or a bad practice?
Javascript will not allow cross posting across domains. PHP can somewhat, but there are problems with it. HTML allows posting but on some browser pop errors.
What are you actually attempting to do? You can not load a registration form into a frame because you can not post it, so it is basically useless. More info please…