This is some code you can inject straight into the url. When you click on the empty link, it will redirect you to the same site you're on, but in a different tab, wait three seconds, and then it will change the url to be https://www.whatismyreferer.com/ and then it will show you what website you were just on. It has been tested. The snippet doesn't work, but that's because there's no window.
var a = document.createElement("a");
a.textContent = "thingy";
a.addEventListener("click", function() {
var referer = window.open(window.location);
var timeout = setTimeout(function() {
referer.window.location = "https://www.whatismyreferer.com/";
}, 3000);
});
document.body.appendChild(a);