Drop a head copy of check-password.4th into /boot and then apply the patch (only the patch to /boot/check-password.4th is required; no other changes are required but you do have to have a HEAD copy of check-password.4th to apply the patch).
NB: The rest of your /boot files can be up to 2 years old but no older.
NB: The test won't work unless your kernel has the following change
https://svnweb.freebsd.org/base?view=revision&revision=273489
Now, put into /boot/loader.conf:
geom_eli_passphrase_prompt="YES"
and reboot.
You should be prompted for a GELI passphrase before the menu (if enabled), just after loading loader.conf(5).
NB: It doesn't matter if you're using GELI or not. However if you are using GELI and a sufficiently new enough release (has SVN r273489) and you entered the proper passphrase to mount your GELI encrypted root device(s), you should notice that the boot process did not stop (you went from loader all the way to login).