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That block currently doesn’t exist in DnD, but even if it did, it wouldn’t help with your app. It would (in that setup) only run once. There are two ways to make a chat. One is just with blocks and the other is with some javascript.
With just blocks, the general idea would be to have a forever block continuing to look at a specific path in the database. Then if there is a change spotted/new entry, dynamically create the “chat bubble”