Spotify, the Swedish music streaming service, is set to announce a new feature that will allow users to pick specific tracks and listen to them on mobile devices for free. That’s according to a person familiar with the matter who was not authorized to speak about the feature. It will be announced at an event in New York on Wednesday. The person spoke on condition of anonymity. The Wall Street Journal reported earlier on the free feature, saying it will mostly serve up music based on the user’s input. The person went further, saying users can pick tracks and put them in a playlist. If the playlist is long enough, Spotify will shuffle the songs and play them back a limited number of times, interspersed with ads. If there are not enough songs in the selection, Spotify will fill in the gaps with other songs, the person said. A Spotify spokesman declined to comment Thursday.