for explaination :
call a script only shows 'non definition scripts'. It will run the code only when it gets called by that action
Definition scripts however run all at start of your game. Putting functions in it will make these functions available anywhere to your game.
When you want to call these functions, just use the "execute a script" action part and write your function call there:
In your screenshot you use a hook to run that function whenever a text get displayed/positioned automatically by the game/engine.
There is no need to call it manually in this case.