Accessing a function from another file (under the same project) in google scripts

Question

So I have two files ImgurAPI.gs and OAuth2.gs and I'm trying to call a function from OAuth2.gs in ImgurAPI.gs (or use a variable either works). I was under the impression that I could just call the function from this post:

Call functions from another "class" / file

However, I get the following error

ReferenceError: getImgurService is not defined

Here's my code

OAuth2.gs

function getImgurService(){
...} 

ImgurAPI.gs

var imgurService = getImgurService(); ...

So how am I supposed to call a function from another file?