I suppose you are talking about those ads that customers get directly to their Messenger. If you are running this kind of campaign, you need to have previous conversations with them. Those ads are going to people whom you already talked to through Messenger. If you don't have any customers you talked to through Messenger, it will not show to anyone. It's worth running the other type first to get messages from customers first, then you can "retarget" them directly in their inbox. If it's not what you mean, I don't know then