Have an extra actor in the database who you'll never actually add to your party. Call him whatever you like. After the NPC asks the question, go to Name Input Processing for that extra actor, and then have a conditional branch -- if actor X is named "whatever," then:
That should work. Tell me if it doesn't, because I haven't tried it myself.
If it doesn't work, though, show me how you set the event up. It'll make it easier to help.