I'm trying to use the following to get an access token for a browser game I'm working on:

var queryString = window.location.hash.slice(1); 
var params = new URLSearchParams(queryString); 
var accessToken = params.get("access_token");

The problem is the queryString and params are empty and the accessToken is null. Is there something I need to set up before I use this code?

Are you trying to get an access token within the Itch app or on the website? I don't think you can use the OAuth method in the app, and on the website you can't get a token for your HTML5 game directly, unless you redirect/open a new page to your OAuth URL and then redirect back to your game.

I tried that but I get the following error:

Refused to display 'https://itch.io/' in a frame because it set 'X-Frame-Options' to 'sameorigin'.