Ad position above message content only show in specific user's post

[Mr Lucky]

Does anyone know how to make content in ad position:above message content only show in posts by one specific user?

Or any other way to achieve putting some content there.

e.g I'm aware I could also use CSS .message-body:before, and can pin it down to a specific post with [data-lb-id="post-509573"] .message-body:before, but how to pin it down to a specific user?

Thanks for you help!
Ad position above message content only show in specific user's post

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