How to make formula field in ZOHO CRM?

I am working with ZOHO CRM. I want to make formula field in "Quotes.Payment Type". The feild is pick list.

Post as payment on Account (SA) = 4 - SO 000001
Post as payment on Account (SA) = 4 - SO 000002
Post as payment on Account (SA) = 4 - SO 000003
Payment already received = 0 - SO 000004
Non selected = 3 - SO 000005