Create Chrome extension - website injection with custom field

Job ID: 37012118

Budget: €6 – €12 EUR

I use Baselinker to create invoices for my business.

The problem is that Baselinker only allows "Price" input which is gross price. I would like to have a custom input field "Net price". When you type a number in "Net price" field, the tax is added to it and it is inserted to "Price field"