I was playing with two proof of concepts a while back:
a) Secured content hosted on AGOL - I could get that working by editing the obfusticated code in "~\cdn\1\jimu-arcgis\index.js"
There is for sure a lot of reasons not to do this, but it was just a test if one could get it working. DONT use this for a real world scenario.
b) Secured content hosted on a Portal instance that is protected by internal Active Directory credentials.
No luck here, I gave up. We are not upgrading a WAB application to ExpBuilder yet.
At this point I would prefer to have a good document / tutorial available before I spend more time on it.