how can I find belnr and knumv in additional data b?

Hi;

I added exstra fields in va01 transaction using program name = SAPMV45A

screen 8903.

I want to bring condition value to my extra field 'sales fields'.

I write code in ' MV45AOZZ' but I can't find vbak-knumv field so I can't calculate new value.

How can I find value of vbak-knumv ?