Grabbing a string from the address bar and adding it to a link?

Hey guys, new here so I'm sorry if I'm posting this in the wrong section.

Was hoping for some help. Not exactly sure if this is PHP or Javascript related so just posting under General.

I am wanting to change a client side link on a website button depending on, I guess it's called a string, that's linked to the address bar.

For example, I have a button that says "Sign Up", directed to Example Domain. If however, someone visits the site with the prefix "www.example.com/?start=Username", I'd like to change the button on the website to then link to "www.example.com/signup?start=Username".

Is that possible? And if so, what do I need to use exactly?

I'm assuming I'm using the wrong words as googling for a solution isn't giving me what I'm looking for.

Thanks for any help.