Use curiosity and a sense of mystery to intrigue your audience and incentivise them into referring.
For example, subscribers could unlock a "mystery" video or other content when they make three referrals. Or you might ship them a "mystery" gift box when they hit 25 referrals.
In general, fewer of your subscribers will be incentivised by an unknown reward. But there'll be a few subscribers who really want to discover what the mystery reward is. And they'll be hooked.