I saw that Jeff replied to my comment on his blog post about registering Field Maps with Portal. Here are the instructions.
1. Open the ArcGIS Portal Directory and login as a member with administrative privileges. The URL format is:
https://webadaptorhost.domain.com/webadaptorname/sharing/rest
2. Navigate to the administrative user’s content.
http://webadaptorhost.domain.com/webadaptorname/sharing/rest/content/users/
3. Under Supported Operations choose Add Item.
4. In the Add Item form, enter:
Title: ArcGIS Field Maps
Type: Application
Type Keywords: Application
Tags: ArcGIS Field Maps
5. Click Add Item.
6. On the User Item page, under Supported Operations, click Share Item.
7. On the Share Item page, set Share with Everyone to True, click Share Item.
8. On the User Item page, select and copy the ID value.
9. Under Related Operations, click Register App.
On the Register App page, enter:
Item ID: This is the ID copied in step 9.
App Type: Select multiple from the dropdown
Redirect URIs: Enter [“urn:ietf:wg:oauth:2.0:oob”, “arcgis-fieldmaps://auth/”,”arcgis-fieldmaps-beta://auth/”]
– This value allows the successful authentication response to be sent to the portal URL.
10. Click Register App. The Registered App page displays the App ID.
11. On the Registered App page, copy the App ID value.
12. Open the ArcGIS Portal Administrator Directory and login as a member with administrative privileges. The URL format is: https://webadaptorhost.domain.com/webadaptorname/portaladmin
13. Navigate to Home > Security> OAuth> Change App ID.
14. On the Change App ID page, enter:
Current App ID: This is the App ID copied in step 11.
New App ID: Enter “fieldmaps”
15. Click Change App ID.