You could add the following to a Script Editor web part on the page or form your expression is needing this library:
That works fine in most cases, but can cause some timing issues if it is not loaded before the expression is executed. Also it will be loaded every time the form or page is opened, which is unnecessary if your expression is not called every time.
So a better solution is to use the following inside a function expression:
var url = "https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.js";
var fileName = "momentjs";
var moduleCache = window.module;
var defineCache = window.define;
window.module = undefined;
window.define = undefined;
window.LoadSodByKey(fileName, null, true);
window.module = moduleCache;
window.define = defineCache;
// Do your stuff here...
This will load the momentjs library before running your code where you can use it as you want (returning "window.moment().format()" is just an example).
In case your wondering: The whole handling of module and define is only needed if we have a js library that works with requirejs, like momentjs. But it won't disturb for other libraries too.
Of course you can use any URL to your js file and change the fileName appropriately.