How to mention external site user

Space Sync for Confluence allows you to mention users on external sites if the destination space allows your space to mention.

Have pages edit mode.
Open the mention dialog by clicking the mention icon on the upper right.
Search user to mention.
Click the copy button.
Paste (with Ctrl + V shortcut or right-click context menu on the page body) mention wherever you want to insert mention in the page body.

FAQ

Question	Answer
When pasting mention, “Unknown user” is shown.	This is a limitation of Confluence. User is resolved when you publish the page. You have to be patient until you publish the page.
Users on the external site are not found.	The destination space admin has to allow your space to mention users in their site explicitly. See Establish space sync with external Confluence sites.
“Unrestricted” link is shown.	This is a bug of Confluence. We hope Atlassian fixes it. https://jira.atlassian.com/browse/CONFCLOUD-72376
Notification is sent to the mentioned user.	Notification is sent to the mentioned user when the page is pushed to the destination space.