How to add a custom column in the invoice table in Zoho Books?

How to add a custom column in the invoice table in Zoho Books?

Hello, 

I recently added some custom fields in the invoice module in Zoho Books which I want to add to the invoice table. I saw in the PDF template settings that it is possible somehow. 

Couldy you please tell me how I can add a new column in the invoice table? 

Thanks, Michael