In most of the cases, airdrops with referral program comes in different structure:
Sample 1: Participants have to invite users in the Telegram group (of that airdrop). Invite people and shows proof to the one who will be assigned to check your credentials. There are different ways to show proofs so it depends on the instruction.
Sample 2: Participants will fill up the form directly at the website and not on the Google form as obviously no codes can be given here. In website registration, there should be a kind of referral code at the end.
Now to minimize our works here, would you mind telling us here what airdrop you are talking about that have a referral program but no way to get a referral link?
Well, for example the one I've linked above (
https://airdrops.io/umc/) But in general, every single airdrop using docs.google I've come across until now (talking about those NOT having a manual referral system inside the form itself, like adding the name of its referrers manually). Obviously, right now, I have a hard time finding one that's still up and corresponding to that criteria. But I've seen a lot of them. I sometimes try to find a referral link everywhere on the website but I can't find any (I know some airdrops don't have referral programs, but still, it happens way too often to be the only reason), so I thought there would a manipulation to do with the docs.google to share a form automatically with its referral link inside or something like that.