'nvram boot-args=...' is there a Mojave alternative for this command?

I have a third party bluetooth keyboard, and am unable to use it to trigger 'safe mode' during boot sequence as the keyboard does not appear to be recognised by the computer until late in the boot process (after login screen appears).

I am having problems logging into iMessage on this computer, and a solution that has been suggested is to restart the computer in 'safe' mode, log in to iMessage, and then restart in normal mode.

It would seem that in the past you could force safe mode boot using the nvram terminal command - specifically setting the variable 'boot-args="-x"' prior to a restart.

In Mojave the terminal nvram command does not recognise 'boot-args' as a valid term - giving the error:

nvram: Error setting variable - 'boot-args': (iokit/common) not permitted

Does anyone know if there is some other way to coerce the machine to start in safe mode other than finding a USB keyboard?

Thanks in advance for any help.