Cronenta has no concept of a test result. It only knows about licence stages (G1, G2, Full G) and dates, never whether a specific test was passed or failed. That’s deliberate: MTO results aren’t something the app has any visibility into.
So there’s nothing special to do. Book the retest the same way you’d book any other appointment. Nothing about that client’s reminders changes, because nothing about their G1 or G2 date changed. They haven’t moved to a new licence stage yet, so the same reminders keep applying until they have.