I want to set up bonus credits of say 100 (every user in my app needs credits to run workflows on the app) that say user A gets if user B buys a subscription with a referral from A.

What's the most efficient approach to do this engineering wise.

Have a referral/[id] link won't work for me, as user B might not straightaway buy the subscription but try out the product first and then go to pricing plans and pick one and then go to billing

In this process the initially shared link by A gets lost and isn't used. A doesn't make credits even if they eventually buy the subscription.

One way is to give a referral code and incentivise B with extra credits for B as well.
But is there any way other than that.