For automatically you need to add a keyfile to a slot in the luks device
# openssl genrsa -out /root/keyfile.bin 4096
# cryptsetup luksAddKey /dev/mapper/extra /root/keyfile.bin
The entry in the crypttab would be like this
extra UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX /root/keyfile.bin luks
I believe systemd after targets work tho I have never tried them Try adding this to mount options