A serial number is a unique 12-digit ID that identifies your software. MYOB provides you with a serial number when you subscribe or trial MYOB. You'll need this number when contacting MYOB customer support.
To see your serial number, sign in and click your business name.
Other places you can find your serial number
you'll find your serial number...
purchased MYOB from the MYOB website
in the email confirming your purchase
created a trial version of MYOB
in the email confirming your trial