how to set usb key bootable

[tech-lists]

Hello,

I'm trying to make a GPT-formatted usb stick bootable and despite looking in
the handbook, I cannot find an example using this. The context is a wholly
freebsd-12-stable system. No dual-boot. The reason I want the usb key is to
experiment with various configs in a safe environment. The key is 32GB, on it
I've made an 8GB swap partition and have installed freebsd to the remaining
space.

It all installed OK but now when I go into the bios and select the key to
boot, it gets looked at then proceeds to load the normal HD environment as it
would if the key was not plugged in at all. 

I suspect something needs to be added in the config on the key to make it
bootable. Can anyone tell me what that would be please? I know that with MBR
one has a flag, somewhere. It's normally turned on as part of the install. I
don't know where though. Is it the same with GPT ?  

thanks,
-- 
J.
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