the feature service I am adding to AGOL is a REST Feature Service published through ArcGIS Server. I'll check the developer tab as you suggest. I can add the service with stored credentials to Portal, and I can add it without stored credentials to AGOL, just can't seem to get it to accept it in AGOL with stored credentials.
Edit: I just tried this again and it worked with no issues, very bizarre. If you look at my original post, I didn't receive the first of those two credentials prompts this time around, so unsure what changed between then and now. Its possible our network has been funky recently but I'm unsure how that might contribute exactly.