wpa_supplicant config for ap with no ssid

[fxkl47BF@protonmail.com]

how do i configure wpa_supplicant for a ap that does not advertise ssid
i normally use

network={
ssid="ap"
psk="passphrase"
id_str="ap"
}

i tried

network={
bssid=12:34:56:78:90:ab
psk="passphrase"
id_str="ap"
}

with no success
i read that a hex passphrase is required
i found references to pbkdf2 to generate it but no details