AlwaysSearchSubreddit.user.js

// ==UserScript==
// @name         Always Search Subreddit
// @description  Checks the box to limit reddit search to the current subreddit
// @version      1.0.0
// @author       heavyLobster2
// @namespace    github.com/heavyLobster2
// @downloadURL  https://gist.github.com/heavyLobster2/7b23d50626e8d3ecc4b423a4d0a7ced9/raw/AlwaysSearchSubreddit.user.js
// @include      *://*.reddit.com/*
// @run-at       document-idle
// @grant        none
// ==/UserScript==
(function () {
    "use strict";
    var box = document.querySelector("input[name=\"restrict_sr\"]");
    if (box && !box.checked) { box.click(); }
})();