It’s actually not hard to find.
Please just follow our very detailed and illustrated with many screenshots step-by-step instructions:
The issue described in the original post won’t happen if you follow instructions item 1.5 “Enter your domain to the App Domains.”
The issue described in the second post won’t happen if you follow instructions item 1.9 “Please enter the URL of your SNAP Plugin “Accounts” page to the “Valid OAuth redirect URIs” field. Click “Save changes”
All you need to do is to carefully follow the instructions.