If you need to add a comment to your trip request, you will do that by going to the Main tab / look on the left side of the screen and click Your Trip Requests / click on the blue trip number to open the request form (my screenshots are from a sample trip) - those steps look like this :

Now that you are inside of your trip request form you need to scroll all the way to the bottom - you will see a button that says "Comments" - click that button and a box will open up - you will see where you type the comment - when finished you must click the "Yes" button in order for your comment to be saved - that process looks like this :

Now that you have added your comment you can see it on your trip request if you scroll to the very bottom and look at the history.