We are testing implementing Portal and running into an issue. Are items that are in a ArcGIS Server that has been federated able to have credentials stored and brought into ArcGIS Online? Looking at this post, this user has the same exact issue I am having:
I would suggest you look into your SSL Certificate - you can test it via https://www.ssllabs.com/ssltest/analyze.html and it will let you know if there are any problems with it. If there are ArcGIS Online won't trust it and you won't see any options to store credentials.
It is possible to bring layers within Enterprise into ArcGIS Online. ArcGIS Server, at the very least, needs to be accessible from the internet (if not Portal as well). The issue in the blog post you linked too is that IWA web-tier authentication is not supported as a way for ArcGIS Online to access content within Enterprise, a built-in account is needed instead.