Solutions
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Lachanophobia's post in Help with Unraid Notification Settings (discord) was marked as the answerI have the answer to my question. When clicking the button "Test", Unraid doesn't use the Discord tag id, but when there is an actual notification, then it works as expected and tags the person.
Not sure if it's possible to tag multiple users, maybe with comma separated user ids but I haven't test it.
The user id format which works is the one Unraid specifies, "4324234234422" (only the numbers)