How to create a serial number in Invoices module in Zoho CRM?

How to create a serial number in Invoices module in Zoho CRM?

Hello, 

is it possible to generate a serial number and output the number in a custom input field with the name "Serial_Number"?

I already used the autonumbering field for another serial number, but I need a second one. 

Thanks for help, 
M.