Using a URL query parameter such as ?username or shorter ?u would be great to use to get the username instead of the "hash" at the end of the URL, which is normally used for going to a specific section of a page.

Basically, to get a query parameter in JS, we can use (Source) :

var searchParams = new URLSearchParams(location.search);
var username = searchParams.get("parametername")

To not break things, using a hash would transform it into a query parameter.