How to count the number of time a participant push a key ?

Hi everyone,
I'm new on this forum.
I'm here because i try to record the number of time a participant press a key when a text is displayed on the screen. So i try to adapt a script i fund here to build this function (my only training in programmation is a 20 hours tuto on openclassroom...). I add the script in the question (in fact in the descriptive text). Here is my attempt :
Qualtrics.SurveyEngine.addOnReady(function() {                                          
 let questionId = "NumberOfPushG" ;                                                    
 document.onkeydown = function(event) {                                                
  let i = 0                                                                            
   if (event.which == 71){                                                             
     event.preventDefault();                                                           
     i += 1                                                                            
     Qualtrics.SurveyEngine.registry[questionId].setChoiceValue(i);                    
       }
   }
});
Of course it doesn't work i would not be here otherwise...
What am i doing wrong ?
Thank you in advance to everyone.