Maintaing the Mandatory Text in Billing Doc. Header Issue

Hi,*

My business Scenario i want to maintain the Header Text as a manadatory because its order related Biling

and also i cant mainatin in the sales order because once production is completed then only i can maintain the text

I know its not not possible in Standard SAP

we can do by using Userexits

Can any one tell me what are the userExits is required to maintain the mandatory for Text in Billing Doc header