Step-by-step guide for ARCGIS ONLINE
From the "My Content" tab, select "New Item"
(Select the "Application" option in the next step)
(Select the "Other Application" option in the next step)
Call it "Clean My Org," or "Backup My Org" (based on which solution you're using).
The Client ID will be listed in the Credentials section in the Item Overview
Now, go to the Settings Tab for the Application Item
Scroll down to the "Redirect URLs" Section. This is where you will configure the Redirect URLs
Click the “Add” button, and add all relevant redirects to this section (if there are multiple ways for your Organization to access CleanMyOrg/BackupMyOrg, you will need to add each different domain).
Note: If you’re using https instead of http or a different port, the Redirect URI will need to be exact.
Finally, ensure you click the "save" button to save these changes.