Well,

From the Terms and conditions, i found:

4. Prizes

By referring contributions worth at least 756 USD in total, the Participant gets one LastuCase for Jolla Tablet.

By referring contributions worth at least 1890 USD in total, the Participant gets one Jolla Tablet (in addition to the LastuCase for Jolla Tablet).

So I thought all that is required is that the referrals refer to any perk in Jolla tablet -campaign. You just need to collect enough dough with your referrals.