Timing of Foe Slayer and Favored Foe (Tasha’s)

Question

Favored Foe is used on hit, and works with Foe Slayer. Does the Ranger need to hit normally first, (without using Foe Slayer), and then on future attacks they can use Foe Slayer? Or, can the Ranger use Foe Slayer to turn a miss into a hit, allowing Favored Foe on the first attack?

Answer 1

With Favored Foe you have to hit the target normally first to be able to use Foe Slayer

Favored Foe says:

When you hit a creature with an attack roll, you can call on your mystical bond with nature to mark the target as your favored enemy for 1 minute or until you lose your concentration (as if you were concen­trating on a spell).

Favored Enemy makes you pick a kind of enemy in advance, and is always "on" against this limited selection of creatures. Favored Foe instead works against any creature, but they only become your favored enemy once you hit and mark them.

As Foe Slayer works only against your Favored Enemy, and the creature is not yet your Favored Enemy when you have not yet hit and marked it, it will not work on the first attack.

Foe Slayer. At 20th level, you become an unparalleled hunter of your enemies. Once on each of your turns, you can add your Wisdom modifier to the attack roll or the damage roll of an attack you make against one of your favored enemies. You can choose to use this feature before or after the roll, but before any effects of the roll are applied.