Our platform automatically uses a "backerclub" referral code for links to your Kickstarter project, so it is not possible (and is unneeded) to supply a custom referral code.
A Kickstarter referral link is one that includes a special "ref=_____" code in the url. In theory, this provides a way to distribute anonymized links to your Kickstarter project page that will segregate referrals back to a known purpose. You can learn more about this on Kickstarter's support center.
Most creators want to use this functionality with their advertising partners, so BackerClub automatically uses a referral url for Kickstarter campaigns.
The link we use will resemble the following structure:
As you can see, we include "ref=backerclub", along with additional url parameters for Google Analytics tracking (see this help article by Kickstarter to learn more about integrating GA with your campaign).
Because our system applies these referral codes automatically, it is not currently possible to supply a custom referral link for your BackerClub advertising, and there isn't any need to. The referrals will work the same way regardless.