I have done this before but cannot remember how I accomplished it. I am trying to create an item in Portal of a AGOL application. Our portal users then need to access the AGOL app. I remember I created an item and then entered in a AGOL credential to be utilized by the Portal users automatically. I can not remember how I exactly did it a year ago. I really appreciate any guidance and assistance. Thanks!
You should be able to add the app via URL to Portal, then enter the credentials when the prompt comes up (ensuring that you select "save credentials")