Don't deny the trip.  Once you deny the trip the requester cannot add the missing information.  Ask for the additional information by doing the following:

Scroll near the bottom of the trip request and Send Email:

Requester email address will auto-populate.  Enter a subject and your message and click Send Now.  This message will be logged in the history at the bottom of the request form.  A copy of the email will also be retained in the Email Audit Log.  As the requester to send the missing information and then open the trip back up and approve/deny the trip.  

Logged in the History:

Copy of email is in the Email Audit Log and may be viewed by clicking on the envelope next to Email Sent By: