Download the latest ugw3 package from https://github.com/Lochnair/vyatta-wireguard/releases and install it on your USG using dpkg -i wireguard-ugw3-<version>.deb.
cd /config/auth
umask 077
mkdir wireguard
cd wireguard
wg genkey > wg_private.key
wg pubkey < wg_private.key > wg_public.key