Custom Invoice Code

How would I go about adding a custom invoice group with format like:

ID : invoice ID,
INV : custom text
CLIENT : initial of my Client,
M : Month when invoice created,
YY : 2 last digit Year.

when created it would look like: 100/INV-BUMA/IV/15
or can i create addional “Identifier format template tags”


The client ID is not available and will not be available because you can still change the client while in draft modus.
Also, the year only supports the long format.

Possible solution: