Step by step
To @mention an external contact in a universal channel first find their full user name.
How to view universal channel members in Slack.
@mentions will only work if you @ their full name as its listed in the member list.
In the above example, I would type and send @Mila Ivanova (with the spaces) in order to mention Mila in my universal channel.
Why does it look like the @mention didn't work on my end?
If you're @mentioning a user on a different platform from you, it might look like you sent it wrong. However, the user experience is optimized for the receiver of the @mention.
The recipient will receive the @mention with the correct highlight/notification.
The sender does not see the @mention "highlighted" properly because the person they are mentioning is not a native user of their workspace or tenant.
In the meantime, we are working hard to optimize the experience for the senders.