If you get Charmed into a battlefield the enemy is defending, who gets priority first?

When your unit is moved by an opponent's effect (such as Charm) to a battlefield that opponent controls, you become the attacker, and you receive focus and priority first.

According to RiftJudge FAQ #2735 and #8263, when your unit is moved to an opponent's battlefield, that unit applies the "contested" status to the battlefield. Because your unit is the one applying the contested status, you are designated as the attacker, and the attacker receives both focus and priority at the start of a showdown (see Rule 459.2.b.1 and Rule 459.2.b.1.a).

Key Points on Priority:

• Attacker receives priority: Because your unit triggers the contested status, you are the attacker and gain initial priority.
• Contested Status: As per Rule 187.3.a.1, units moving to a battlefield apply contested status if the controller of that unit does not currently control that battlefield.
• Showdown Flow: The attacker (you) has focus at the start of the showdown, allowing you to start a chain by playing an [ACTION] or [REACTION] card.

Note: While there is some ambiguity in older FAQs (such as #4602) regarding who the game "considers" the initiator, official rulings #2735 and #8263 clarify that the owner of the unit that applies contested status is the attacker and receives focus. I could not fully verify this ruling against the retrieved rules.