You can get this bounty poster in the sheriff’s office in Valentine. While her bounty is small and you have to recover her alive, this amount can be helpful very early in the game, especially if you’re not doing any treasure hunting yet.
The target is hiding under an outcrop along the Dakota River, south of Cumberland Falls. Head there and confront the target and her lover. She will stab the poor guy in the neck, which you can prevent if you act fast enough and shoot her hand using Dead Eye.
She will attack you with her knife but switch to your lasso and Arthur will automatically tackle and hogtie her once she’s in melee range. Pick her up and stow her on your horse then ride back to Valentine.
There are no other bounty hunters after her head so you’ll have a smooth trip back to town. Upon arriving, just pick her up again and drop her inside the jail cell. Pick up the reward from the sheriff’s table. Mission complete!