From the Referral page, tap Share to send your invite link through whatever sharing options your device offers (like your phone's regular share sheet), or the link will be copied straight to your clipboard automatically if your device doesn't support that share sheet.
Make sure whoever you're inviting signs up using that exact link (or by entering your referral code when prompted at sign-up) — referrals can only be counted if the link or code is applied during account creation, not after the fact.