Text entry simulation

I'm looking to create an interactive system simulation where the screen changes when a letter is typed in. 

The example I have is that the user needs to choose a country from a drop-down. The drop-down is so large that it has a text box above it which enables the user to narrow down the search by typing in the first few letters. 

The image on the left shows the screen before the letters are entered and the image on the right shows the choices after. 

I've tried setting the trigger to when the user presses a designated key but that doesn't work as it needs to click something to submit it and the same when I set a text entry.

Any help would be greatly appreciated.