Thanks anbarasu chinna!
I've figured how APP-ID looks like, not long after my post, but new issues come up.
I can successfully get Shin-Nil's AdMob demo running on my mobile, but when I integrate Admob in my own app, adb logcat returns:
"AdMob Java Singleton not found. This plugin will only work on Android."
every time when I remote debug my app over USB.
1) I used my own app-id in manifest.xml file.
2) from Shin-Nil's demo Main.gd, I simplfied it to something like:
get_tree().connect("screen_resized", self, "_on_resize")
func loadAds() -> void:
so I can get banner shows up as soon as my app get ready.
still no luck to make it work on my own app.