This Stranger Mission is only available from Chapters 2 through 6, after which, it will become available in the epilogue. Saint Denis is a town located in the southeastern corner of the map and is the location for this very short Stranger Mission. Using your map, locate the tailor on it, which is just above the "T" in Saint. To the west of the tailor is a smaller building that has a small part jutting out on the left side. This Stranger is located on the street corner right there and only seems to appear during the day.
Once you find her, approach the lady and she will talk about how she's collecting money for building an expansion to the Quincy Harris Memorial Hall. She asks for a donation of $20, which will give you some Honor should you contribute. You also have the option to steal from the lockbox next to her, but this gives you a huge hit to your honor and you will have to deal with any lawmen that come your way. Donate the money and you will see the expansion constructed in the epilogue.