mnt-community-sig.user.js

// ==UserScript==
// @name         MNT Community Simple Signature with Observer
// @description  Adds a predefined signature to forum posts on https://community.mnt.re
// @version      0.1
// @author       173duprot
// @match        https://community.mnt.re/*
// @grant        none
// @run-at       document-end
// ==/UserScript==

(function() {
    'use strict';

    const signature = "\n\n---\n\“There’s no forum signatures on this site so I’ll \[do it myself\]\(https://gist.github.com/173duprot/a86b4780b9a0e0785e066a3e04746a93\)\”\n\t- 𝚋⃥̸⃝ 𝚘⃥̸⃝ 𝚡⃥̸⃝ 𝚢⃥̸⃝\n";
    const selector = 'textarea.d-editor-input[aria-label*="Type here"]';

    const appendSignature = (textarea) => {
        if (textarea && !textarea.value.includes(signature)) {
            textarea.value += signature;
        }
    };

    // Initial check
    appendSignature(document.querySelector(selector));

    // Observe future additions to the DOM
    const observer = new MutationObserver(() => {
        appendSignature(document.querySelector(selector));
    });

    observer.observe(document.body, { childList: true, subtree: true });
})();