The Seamless SharePoint Add-In requires an authentication using the Microsoft Identity platform. The Microsoft Authentication Library (MSAL) enables developers to acquire tokens from the Microsoft identity platform in order to authenticate users and access secured web APIs.
This authentication requires that you
- allow pop-ups from [*.]sharepoint.com
If your browser security settings prevent pop-ups, the sign-in will fail and a corresponding message with a button indicating "Click here to proceed" will be displayed.
To avoid the "sing-in failed" message and the additional click on the "Click here to proceed" button, you can allow the authentication pop-up for Seamless as follows:
Or allow pop-ups in general: Open browser Settings
Go to "Privacy and security"
Turn "Block pop-ups" off