If I reveal Here to Help on a battlefield the I control during a showdown can I play units from my hand without ambush to that battlefield with the reduced cost?

Yes — the permission comes from Here to Help itself, not from Ambush.

Here to Help reads "You may play a unit from hand to a battlefield you control, reducing its cost by [3]." That effect is what lets you play a unit during a showdown; the unit doesn't need Ambush or any other keyword (compare FAQ #3310, which notes units generally can only be played during a showdown when an effect specifically allows it — Here to Help is that effect). The played unit finalizes, enters the battlefield, and participates in the showdown.

Two constraints: the reveal itself needs legal timing (Hidden grants it [Reaction] while facedown per Rule 811.6, so it works in a Closed State, and its own [Action] covers an Open showdown state), and the unit you play must go to the same battlefield where Here to Help was hidden (Rule 811.1.d.3) — which is exactly "that battlefield you control," so that's satisfied. Cost is reduced by [3].