This script can be modded to add some text whenever a key is pressed. DIY whatever satisfies you, you can change keycodes by replacing "13" and 'keyup' can be replaced by 'keypress' & 'keydown' events whichever satisfies your need

Enter_text_whenever_pressed_enter.js

// ==UserScript==
// @name         Enter "mnemonic" on pressing enter
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://quizlet.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    
    $(document).on("keyup", function (e) {
    var code = e.keyCode || e.which;
    if(code == 13) { //Enter keycode
   //Do something
     var focused = document.activeElement;
    focused.value = focused.value + 'mnemonic:'; 
 }
});
    
})();