Setting an autoindex for new items entered by form

Setting an autoindex for new items entered by form

I am very new to zoho and still trying to find my way around, but could anyone shed light on how to have an autoindex entry starting with the year of the date set in the form so that each entry can be indexed using the format:
YYYYnnnn where nnnn is a four digit index number.
Any help is much appreciated. Loving zoho so far.