You can contact the helpdesk and they can add the referrer in still i believe, dont think there is a way online to add in after the event.
Timescales and what to do if missing for the cashback in the following FAQ
https://www.plus.net/home-broadband/faqs/cashback-and-vouchers/
How do I claim my Plusnet Rewards?
If you've signed up to a cashback or Multi-Choice voucher offer, you can claim your Plusnet Reward in just a few easy steps:
You'll get an email sent to the address you provided when you signed up. You'll normally receive your Plusnet Reward email within 10 working days of your first service being up and running.
Open the email and click on the link - it's valid for two months from the date we sent it. You'll then be directed to an online form. Fill in your details into this form and then click 'submit'.
You'll then see a confirmation screen - your Plusnet Reward is on its way!
You should receive your reward within 30 days of receiving confirmation it's on its way. It will be sent either via post or email, depending on the type of reward.
Sometimes we offer Plusnet Rewards automatically and you won't have to claim your reward as set out in these FAQs. However, this is normally only the case when you sign-up via a comparison site and we'll make it clear in the "legal bit" when you sign-up. If you're not sure then we recommend you get in touch with us on 0800 652 6581 (you'll need to have your Plusnet account number or username handy).
