This is valid only for the rolling release. 1.2 does it differently. Using the fake data provided you would configure like this.
set interfaces wireguard wg0 address '10.0.0.5/24'
set interfaces wireguard wg0 description 'fake-config'
set interfaces wireguard wg0 mtu '1420'
set interfaces wireguard wg0 peer fake-peer address 'vpn.example.com'
set interfaces wireguard wg0 peer fake-peer allowed-ips '0.0.0.0/0'
set interfaces wireguard wg0 peer fake-peer persistent-keepalive '15'
set interfaces wireguard wg0 peer fake-peer port '12345'
set interfaces wireguard wg0 peer fake-peer pubkey 'e2YIp/0hLXpUFA2pBqVvR635OLF8dP5DEpgXnSx5rn4='
set interfaces wireguard wg0 private-key 'wg0'
You place your private.key and public.key files containing their respective values at this location
/config/auth/wireguard/wg0