I see that I can configure when I first launch to use Portal, where is another place this can be done after logging on to Pro. There needs to be a link somewhere in the interface, perhaps as part of the Portal Area on the Properties Section. Currently, there is a link where I login to AGOL.
There is no other place to make this change after you've started the software, and my understanding is there won't be a place within the software to do this, e.g. on the Licensing or Portal pages.