Use the below code:
Qualtrics.SurveyEngine.addOnReady(function() {                                          
 var i = 0;                                                 
 document.onkeydown = function(event) {                                                
if(event.which == 32){                                                             
     event.preventDefault();                                                           
     i++;                                                                            
     Qualtrics.SurveyEngine.setEmbeddedData("count",i);                    
   }
 }
});

Thank you very much for your reply!
I've added that code to the JS of the question (see attached printscreen), and I've previewed the survey several times, but when I export the dataset, it doesn't seem to have sized a variable with the number of spacebar clicks.
Have I missed something?

Have you created embedded data - "count" at the start of survey flow?

Hello,
Did you ever get this working?
I am trying to do something similar but with the "D" key. See below:
I have created an embedded variable "count" at the start of the survey flow, but nothing seems to be registering when pressing "D". Could you tell me exactly how the embedded variable should look in the survey or let me know if you see any errors?
It might be important to note that I am trying to register the number of key presses while the participant watches a video in full screen, hence the full screen code.
Thank you for your help!