Set nick name via URL query parameter

I have visitors enter their name in a form. On send, they are redirected to the page containing the chat window. To avoid having them enter their name a second time, I would like to pass on the name they entered in the form via a query parameter. Visitors are anonymous, no user account is created.

Example:
say the chat window is on the page xyz.com/chat
when the page is called like this: xyz.com/chat?user=myname
the user name in chat should be set to “myname“