The process to use SSO differs based on whether SSO Authentication is toggled on or off in the selected authentication setting

To use SSO when SSO Authentication is toggled on:

You are signed into the Excel Add-in.

To use SSO when SSO Authentication is toggled off:

After you sign in with SSO for the first time, the add-in appends ?svcId=auth to the SSO Friendly URL. Do not remove this, it's required for technical reasons.