Two Questions.

I know you put flag 10 or 9 on a tile and set an under combo, then set room type:secret item, secret item. When you make what's under the 9/10 flag change to the under combo, the secret item appears there.

You could: place a whistle flag on one tile, a secret flag on another.
Set the chest combo as secret tile. It shuold be combo type slash -> item. Hold CTRL + click on the secret tile again, then set flag 10 (armos->item). Then when you whistle on the whistle spot the secret is activated and the chest appears. When you slash the chest (it's a slash->item combo) it turns into the under combo (which you set as an open chest). But there's a #10 flag on it so the secret item appears on the under combo.

I know this is not quite what you asked but I'm not that much of an expert, I'll post back if I find what you wanted; I'm making my first quest myself and am looking for the same thing.

well thanks for the help then i really appreciate it.

The general way to give items etc. is by secrets. Whistling on a whistle tile, burning a burn flag etc. triggers all secrets on that screen. A secret can be a chest appearing as above or stairs (which are set to lead to a "cave room", those rooms where you walk down a ladder to get an item and there are usually 4 keese [bats]). There is as far as I know no way to place an item directly anywhere on screen on triggering a secret except making 2 identical rooms and warping link to the second when the secret triggers, and placing the item as regular item on the 2nd screen. That's why most quests use cave rooms.

good explanation, thanks i get it now.