You can use the 4th parameter for timeout:

if messagebox('Please click OK to shutdown ',0 + 48 + 0, '', 6000) = 1
  wait window 'ok'
else
  wait window 'timeout'
endif

>This messagebox is called from a form button
>
>
>How can I do to insert a timer between the messagebox below and the
>quit command so the costumer have to wait some seconds after
>he hits OK button and before the quit command is executed?
>
>If possible also show the seconds count donw
>
>
>================================================================
>= MESSAGEBOX("Please click OK to shutdown ",0 + 48 + 0, '' )
>
>
> QUIT
>
>================================================================