The fstab(5) file can only be used with filesystems mounted via mount(8), which requires statically build mounting applications in /sbin/mount_XXXX.
The usmb package's mounting program is /usr/local/bin/usmb, not mount(), so fstab() cannot be used. Instead, create an /etc/rc.local shell script to run usmb at boot. Per rc(8):
Code:
rc.local is executed towards the end of rc (it is not the very last as
there are a few services that must be started at the very end).
Normally, rc.local contains commands and daemons that are not part of the
stock installation.
A skeleton shell script you can adapt may be found in /etc/examples/rc.local.