Do defend and attack triggers that target something need to be decided before a Reaction?

Yes, the targets for "When I attack" and "When I defend" triggers must be chosen before any player can play a reaction.

According to the Showdown Timing Flowchart and the associated RiftJudge FAQs, the process for these triggers is as follows:

1. Placing Triggers on the Chain

When a combat showdown begins, the triggered abilities are placed on the chain as follows:

• Step 1: The attacker (who has focus) places all their "When I attack" triggers on the chain. They choose the order in which these triggers are placed.
• Step 2: Any non-defender players (in turn order) place their triggers.
• Step 3: The defender places all their "When I defend" triggers on the chain. They choose the order in which these triggers are placed.

2. Making Choices (Targets)

When a triggered ability is added to the chain, you must make all required choices for that ability—including choosing targets—at the time it is placed on the chain (Rule 402.1). An ability cannot be placed on the chain if legal targets cannot be chosen for it.

3. Reaction Window

• Once all triggers are on the chain and all choices (including targets) have been made, the state becomes Closed.
• The defending player receives priority first to play a reaction, followed by the attacking player.
• The chain then resolves in LIFO (Last-In, First-Out) order, meaning the last items added (usually the "When I defend" triggers) resolve first.

Summary: You cannot wait to see if an opponent reacts before deciding who your ability is targeting. You must lock in your targets during the initial trigger-stacking phase before any reactions are permitted.

This is my interpretation based on Rule 402.1 and the established Showdown timing flow. No official FAQ exists for this specific "decision order" phrasing, but it is explicitly required by the core rules for placing triggers on the chain.