A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Edwald,
Thanks for writing back and updating the information with us.
As per the further updates, we have searched the SharePoint Online site proxy address but we couldn't find any Microsoft KB about it, where we can only find the SharePoint Online Proxy address for the user via Exchange Admin center. Exchange Online Admin center> Recipients> Mailboxes> Search for the user and double-click on the user mailbox> Email Address
However, to get the detailed information about the SharePoint Online site proxy address, we recommend either contact the related development team directly through creating the support ticket from the Office 365 admin center or post your concern in the Ask a question - Microsoft Q&A to get the help from the developing experts.
Thanks for your cooperation.
Sincerely,
Mia | Microsoft Community Moderator