Replace <VDOM_NAME> with the name of a VDOM. It has to be case sensitive.

config vdom
edit <VDOM_NAME>

Example

FORTI $ config vdom

FORTI (vdom) $ edit EXTERNAL
current vf=EXTERNAL:1

FORTI (EXTERNAL) $ 

config system admin
    edit <username>
        set ssh-public-key1 "<key_type> <key_value>"
    next
end

Example

FORTI (EXTERNAL) $ config system admin

FORTI (admin) $ edit "foobar"

FORTI (foobar) $ set ssh-public-key1 "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEkNwIWR0Rqh1OLcYzkUuXWWqHynHtm47GAMTnOPoFLP"
SSH key is good.

FORTI (foobar) $ next

FORTI (admin) $ end

FORTI (EXTERNAL) $ 

Source: FortiGate / FortiOS: Administration Guide: Public key SSH access