How can I insert defined question id in js?

You can use below code:
var qid = this.questionId;
var res = (document.getElementById('QR~'+qid+'').value);

I should add that if I hard code in the QID (e.g QR~QID2), it works.
For some reason it cannot retrieve value when it has to calculate the question id. I should also add that I have also tried defining a new variable equal to what you suggested in the bracket. And that variable correctly outputs QR~QID2. So your syntax is definitely correct. But if I insert that variable inside getElementById(var).value it does not update res.
thanks for your help again!

Thanks that worked perfectly!!
I have a follow up question (let me know if I should post this as a new thread-- I'm new to the forum so not sure about all the rules).
My code is slightly more complicated - to verify the response I need to create a custom next button, and put in the code when that custom button is clicked, and not the usual addonPageSubmit. My code is below.
It works fine as long as I comment out Qualtrics.SurveyEngine.setEmbeddedData("res", Number(document.getElementById('QR~'+qid+'').value));
With that in, it has the same problem as before, it does not update the embedded field. (I can hard code it as QR~QID22 and all is well again). Do you think it is because of the custom button function?
Sorry for the many questions, and thanks very much for your time!
Qualtrics.SurveyEngine.addOnReady(function()
{
jQuery('#CustomNextButton').on('click', function() {

var res =Number(document.getElementById('QR~QID22').value);
var qid = this.questionId;
Qualtrics.SurveyEngine.setEmbeddedData("res", Number(document.getElementById('QR~'+qid+'').value));

if (res ==correct) {
  jQuery('#NextButton').click();
  jQuery('#CustomNextButton').hide();
} else {
  /* This is where you set your error message text */
  var errorMsg = "Please enter the correct code";
  var x = document.createElement("DIV");
  var t = document.createTextNode(errorMsg);
  x.className = "custom-warning";
  x.appendChild(t);
  document.getElementById('Questions').parentElement.appendChild(x);
jQuery('.custom-warning').css("background", "pink");
jQuery('.custom-warning').css("color", "red");
jQuery('.custom-warning').css("font-size", "12px");
}
});
});


Thank you sooo much! works like a dream now!
much appreciated!