It's easy, sign up as a "business" when creating your billing address for the current order on Weerg
Once the quote has been completed, you will be asked for the billing data through the address interface, after selecting a Business or Public Administration you will find the dedicated fields to fill in.
At any other time, simply go to the Addresses page, click on the "Add Address" or "Edit Address" button and choose the desired "Billing" address type.
You will find copies of all invoices in your Orders Page.