Referring a business works the same as referring any customer. Just send them your unique referral link.
Every Octopus customer has one: it looks something like http://share.octopus.energy/adjective-noun-number. You’ll find yours a little way down the home screen of your online account dashboard.
If you have a residential account (or one for a different business), use that account's referral link to sign up your business. That means £100 for you and £100 for - well - you again!