Dependee sets value of dependent

I am trying to make a drop down menu field value set a second field as visible & contain a value
I have been successful in show/hide the second field but not inserting a value. i noticed the "filled with a value" option and placed text into the corresponding field for value, but the second field shows but without the value insert

situation:

dependee: country (dropdown menu)
dependent: flag (text field or dropdown menu)
I wish for the chosen country (dependee) to change the value of flag <img> as mentioned "filled with a value" option may do this but i can't seem to get it to do anything.

I know the user could do this manually but i see no reason for them to input data twice.