Steam Key Quick Activator

quicksteamactivator.user.js

// ==UserScript==
// @name         Steam Key Quick Activator
// @namespace    http://pedrofracassi.me/
// @version      1.2
// @description  Activates Steam Keys Quickly!
// @author       Pedro Fracassi (http://pedrofracassi.me)
// @match        https://store.steampowered.com/account/registerkey?key=*
// @grant        none
// @run-at       document-end
// ==/UserScript==

'use strict';

if (document.getElementById('product_key').value != '') {
    document.getElementById('accept_ssa').click();
    document.getElementById('register_btn').click();
}

quicksteambookmarklet.js

javascript:
var selection = window.getSelection().toString();

if (selection) {
  location.href = 'https://store.steampowered.com/account/registerkey?key=' + selection;
} else {
  location.href = 'https://store.steampowered.com/account/registerkey?key=' + prompt('Insert Steam Key');
}

@Looke116 did you do it like this?

Just as a small quality of life change for the bookmarklet, you can check for null from the prompt to abort when clicking "Cancel" or Esc.

result = prompt('Insert Steam Key');
if(result != null){
	location.href = 'https://store.steampowered.com/account/registerkey?key=' + result;
}

Nice. Added validation of key format:
https://gist.github.com/Haoose/c081fa5dd95c4d2139f3e3bb310598cb

If you use this for the bookmarklet, it should open the activation page in a new tab (helpful for IndieGala, which keeps losing my place):

javascript:
var selection = window.getSelection().toString();
var loc;
if (selection) {
  loc = 'https://store.steampowered.com/account/registerkey?key=' + selection;
} else {
  result = prompt('Insert Steam Key');
  if (result != null) {
    loc = 'https://store.steampowered.com/account/registerkey?key=' + result;
  } else {
    loc = 'https://store.steampowered.com/account/registerkey';
  }
}
var popup = window.open('about:blank', '_blank');
popup.location = loc;
null;

(only tested on Chrome)

Instead of the last line being "null;", change it to "void(0);". This will stop the browser from redirecting the page you're on to a page that just says "null;" - at least in Firefox.