When I attack a battlefield with no units and I use a Moonfall, will the unit I target to move be attacking or defending?

According to the RiftJudge FAQ #9407 and FAQ #9339, when you move a unit to an empty (uncontrolled/unoccupied) battlefield, you apply the "Contested" status to it. Because you were the player who first moved to the battlefield and applied the Contested status, you are the attacker, and the enemy unit moved to that battlefield becomes the defender.

Here is the breakdown of the sequence:

1. Initial Move: You move your units to an open battlefield. You apply the Contested status, becoming the player who initiated the showdown.
2. Playing Moonfall: You cast Moonfall to move an enemy unit to that same battlefield.
3. Combat Transition: Once opposing units are present at the same battlefield, the ongoing showdown transitions into a Combat Showdown (Rule 447.1).
4. Designations: Because you were the player who initially applied the "Contested" status to the battlefield, you retain the designation of Attacker. The enemy unit moved by your spell becomes the Defender (RiftJudge FAQ #9407).

Note: This is based on the principle that the attacker is determined by the controller of the units that first applied the contested status, not by which player played the spell that moved units into the battlefield.