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: