Take Aim

Action
— Requires a Ranged Weapon
— Requires Concentration

Spend your Action to take your time and improve your aim at a target. Choose a target in sight and in range of your weapon. On your next action/turn, you gain Advantage on your attack roll against that target.

If you lose Concentration due to taking damage, the action fails and you lose the advantage.

If, on their turn, the target starts to move out of sight/range, you can make the attack prematurely as a reaction. This forfeits the aim advantage, and the attack is made normally.