The most tactful way to do this is to first do a search yourself. For intance, if you need to arrive for a dinner at 4:30pm, and you know that a United (or whatever your airline of choice is) arrives at 4:10, ask them, "Can you see anything that arrives around 4:00?" Invariably, they will say, "Yes, the United flight gets in around then. Would you like me to book that for you?

The same can be done for the return. You've done the search on a general website like travelocity, so you know what is around the same time. You ask to leave around the time the next United flight goes, and usually, they will book you on United.

After they book your ticket, just get your record locator number. Then call United or pull up y our reservation on the internet and add your FF#.