It's not an option in your AirPort Utility. AirPort Utility only allows you to share the hard drive using the Disks tab (I'm sure you've seen it snooping around in there). Just make sure you've enabled file sharing, make sure you can mount and browse the hard drive once it's attached to your AEBS, then start Time Machine and choose the AEBS attached hard drive as the target volume. Keep in mind that it's not officially supported by Apple, so they will not help you if you have issues with it. And if you have any additional questions, search here or post if you can't come up with an answer and somebody will be sure to provide help. I've done this using an external attached to my Time Capsule and it worked, but AFAIK it should work for your AEBS.
Edit: Also, be sure to use ethernet to do your initial backup. It cuts down on time significantly. Subsequent backups are fine over WiFi since it's only backing up whatever has changed since your last backup, but the first should always be done wired since it will almost always be a significant amount of data being backed up.