Print billing document by providing document number as parameter separately

Question

Hi experts,
We are aware that we can print billing document from VF03.
My client want a separate program which takes document number
as input and produces the same form which we find in VF03.
I copied the form to a zform.
I tried giving as separate import parameter PARA_BIL_NUMBER
in the form interface.
Initialize IS_BIL_INVOICE-HD_GEN-BIL_NUMBER toPARA_BIL_NUMBER
But this throws an error message saying that value of
IS_BIL_INVOICE-HD_GEN-BIL_NUMBER cannot be changed.
Can some one suggest the best method to acheive this.
Thanks
Joshi

Answer 1

In a previous assignment, all custom forms I created could be called from an application transaction or from stand-alone transaction code or Sx38.
In SAP's forms driver programs, you have an entry point....the application enters the program there supplying some value.  In my customer version for stand-alone execution, I had a start-of-selection routine that got the document number from the screen parameter, did the necessary setup, such as obtaining the form name, or reading NAST, etc., and passed that document number and any additional fields (named in using parameters) to the entry point subroutine.  Or, SAP entered at the entry point subroutine, supplying NAST, etc.
From that point on, the program essentially is the same.