Find the Current Record

Hi All,

How to find out the current record present in sales order based on this record I want to add the new line of item charges automatically and I want to write the code by creating the function in code unit 80 and want to call it for the post button because when I hit the post button the new line of item charges should be added automatically

for example 1st line as item and it's document no. is 1900
and 2nd line as charge(Item) and it's document no. is taking as null

please suggest me to achieve this