You don't need to do it for most IA programs, some have recently started asking you to do it though - ie: BetVictor - and others you may need to do it for tax purposes if you're VAT registered (or similar).
Anyway, just go to:
My Account > Generate Payment Invoices
In the left hand sidebar. Select the month(s) you want to invoice for, add a PO number and click submit. Invoice will appear at the top of the page.