If I am attacking, can I use ambush unit, since I now have units at that battlefield?

Yes, you can use an Ambush unit to join an ongoing attack.

According to Rule 822.1.b and Rule 822.1.c, Ambush is a passive ability that expands the valid locations where a unit can be played. Specifically, it allows you to play the unit as a [Reaction] to any battlefield where you already control units.

How it works during an attack:

1. Valid Location: Because you have units at the battlefield (the ones currently attacking), that battlefield is a valid location for you to play an Ambush unit.
2. Reaction Timing: Ambush grants the unit the [Reaction] keyword while being played to such a location. This allows you to play the unit even while the game state is Closed (such as during the initial chain of a showdown or while other reactions are being played).
3. Joining the Combat: Once the Ambush unit enters the battlefield, it becomes part of the ongoing combat. If the unit has abilities like Assault, they will become active because the unit is now present at the battlefield and designated as an attacker (provided it meets the requirements for that designation).

Important Note: You do not need to control the battlefield itself; you only need to control units at that location to satisfy the condition for Ambush.

This interaction is confirmed by the 2026-03-16 Patch Notes, which explicitly describe using the Ambush keyword to land a unit at a contested battlefield to utilize its Assault ability during combat.