Detail: 
1. add the rd.luks.key into /etc/default/grub
GRUB_CMDLINE_LINUX="rd.luks.key=/keyfile:LABEL=KEYDISK"
2. add the drivers into the initramfs by editing the /etc/Dracut.conf
Add_drivers+=" vfat nls_cp437 nls_iso8859-1"
3. put the file into the KEYDISK with label, a text file with the passphrase.

Then reboot,
The Plymouth passphrase also prompted.

Also I try to turn on the debug shell by adding rd.shell=1 into the
GRUB_CMDLINE_LINUX...
Also I could not get the shell command.