Unlock GELI on boot with USB stick

My system is encrypted with GELI and uses the AUTOZFS partition schema. I can successfully boot up without issues.

I would like to make a backup system that boots up with a USB key, and then after booted, I will remove the USB key. The backup system is completely headless, but if needed, I could attach a monitor. I do not keep it on all the time and only turn it on when doing backups and don't want to mess with plugging in a keyboard and trying to guess when to enter the password, etc.

For me to be able to boot the system presently, I must first enter the GELI passphrase. Is it possible to configure it to look for a USB drive and use that key?