The choice of creature type is made when the enters-the-battlefield ability resolves.

You must choose an existing _Magic_ creature type.

Creature cards not on the battlefield and creature spells are not affected.

Replacement effects that modify creatures of a certain type as they enter the battlefield will apply (or not apply) before you apply Xenograft’s effect. For example, if Warrior is the chosen creature type and you control Bramblewood Paragon, a Runeclaw Bear would not enter the battlefield with an additional +1/+1 counter.