In the initial showdown chain I'm attacking so I move my unit to the battlefield, the when attacking and when defending actions go onto the chain my opponent does not play a reaction. Does that initial chain resolve automatically or can I still react and play a spell that will trigger before the on defend trigger?

The initial showdown chain does not resolve automatically; instead, it follows a specific priority process once the triggers are placed on the chain.

According to Rule 459.2.d.1 and RiftJudge FAQ #6641, when a showdown begins:

1. "When I attack" triggers are placed on the chain first.
2. "When I defend" triggers are placed on the chain last.

Because the "When I defend" triggers are placed last, they are the topmost items on the chain. Rule 340.4 and RiftJudge FAQ #6641 dictate that the controller of the topmost item on the chain (the defender) receives priority first.

How you can react

Even though you are the attacker, you cannot simply force your spell to trigger "before the on defend trigger" if the defender has priority and chooses to let the chain begin resolving. Here is how the sequence works:

1. Triggers are placed: All "When I attack" and "When I defend" triggers are added to the chain. The state becomes Closed.
2. Priority starts: The defender, as the controller of the topmost item, gains priority first. They can play Reaction speed spells or pass.
3. Alternating Priority: If the defender passes, you (the attacker) gain priority and may play a Reaction speed spell. If you do, your spell goes on top of the chain (becoming the new topmost item), and the defender gets priority again.
4. Resolution: Once both players pass priority consecutively, the topmost item on the chain resolves (LIFO - Last In, First Out).

Crucially: You can only play a spell "before the on defend trigger" if you are able to add it to the chain above that trigger. If the defender passes priority to you, you can play a Reaction spell, which will place your spell on top of the "When I defend" trigger. That spell will then resolve before the trigger.

Summary: You cannot force this automatically; you must wait for the defender to pass priority to you. If they pass, you can play a Reaction spell to place your effect on top of the chain, ensuring it resolves before the "When I defend" trigger. You cannot play Action speed spells at this time; only Reactions are allowed during the initial trigger chain (RiftJudge FAQ #6677, #8384).