The first transaction can be anything – wallet top-up, regular proxy, or a scraper plan. Just remember regular means not enterprise. The referral program does not apply to any enterprise plan. One more thing – the first transaction is the first purchase ever made, not the first one for a different plan. E. g. if your friend bought a residential proxies Starter plan and then bought a regular mobile proxies Micro plan – Starter plan is their first transaction. Micro plan is the second.