Ok. I think I'm understanding better now.

After requesting the refresh token with the code, my url only redirects with the same code. This is difficult to explain, but I am able to return all data except the refresh token.

So it sounds like the code is coming back with the redirect and things are stopped right there. Assuming code is returned correctly, then we should be hitting this:

    let code = req.query.code;
    if (code) {

Your POST looks good to me which means you're getting here:

            //let qs = "access_token=" + results.access_token + "&refresh_token=" + results.refresh_token + "&username=" + results.username;
            res.render('customize', { title: "appname", username: results.refresh_token, refresh_token: results.refresh_token });
              // results.refresh_token returns undefined?
 

the res.render is sending you to http://localhost:3000/customize?code=longcodeherebutpopulatedwithactualcode

This is where there's some additional confusion. The route described is:

router.get('/', function(req, res, next) {

However, the url ends up being:
http://localhost:3000/customize?code=longcodeherebutpopulatedwithactualcode

Somehow, we're getting to another route. res.render should just be rendering a template, right?

What does the /customize view looks like. Is it also handling code? It just looks like some weird thing is happening where it's not sending the right data, but code again. I'm trying to see why that is, but I don't see why the customize template would change the URL.