I think I found a solution to your problem. But I must admit it was not evident. In apps scripts documentation you've got everything to create a prefilled url.
BUT for that you need to have a ItemResponse element and I didn't found any explaination to build one. The trick is when you've got an item you can get a ItemResponse from it if you get it as a defined type "asTextItem()".
Best way to understand it, is to watch the code below:

function getPreFilledItem(){
  var form = FormApp.openById("YOUR_FORM_ID"); 
  var items = form.getItems();
  var itemOfInterest;
  for(var i in items){
    if(items[i].getTitle()=="YOUR_QUESTION_TITLE"){
      itemOfInterest=items[i];
    }
  }
  Logger.log(
    form.createResponse().
    withItemResponse(itemOfInterest.asTextItem().createResponse("PREFILLED_TEXT")).
    toPrefilledUrl()
  );
}

Hoping this will help you,
Harold