The text of the tweet must contain the tags to be added.hashtag and the OpenStreetMap
The normal usage would involve sending a location enabled tweet i.e. using GPS location on your smartphone, and allowing twitter to send your location. This may present some accuracy problems since generally a smartphone's GPS is not accurate enough for placing POIs particularly in urban environments. There is an alternative, which is to enter a lat, lon in the tweet text (like 50.488, 30.536) to set precise location.
OpenStreetMap edits are made immediately into the OSM database under your username. You need to authorise this on the site first (It uses OAuth)