Your referral code is sent to you by email when you registered on the Helpling site. You can simply search "Want $20 off your next cleaning?", then you will see all emails contain the referral code.
You are also able to view your referral code by signing into your Helpling customer account. You find the code on your home page in the 'Refer A Friend' section.
If you have your referral code, pass it to a friend and ask them to enter it when they make their first booking.