Add a record in invoice through Custom Button

Hi folks,
I want to add a record at RMS invoice using my custom button. The scenario is, I have created a plugin that will input the invoice quantity from the user in RMS and after this it sends the request to the server for authentication. If server response is OK then I want to insert a record in the invoice. My question is: how can I add the record on the invoice? Do I need to insert directly in to SQL Database? If so then what are the tables that I need to play with?

I would be really be thankful for you.