Event-triggered messages are only available for Email, SMS, Push, and Webhook messages. In-App messages and A/B tests operate off a different API that leverages a separate Messages backend. This separate API currently supports delivering messages off of People Properties only.
Create People Properties based off Events
It is possible to create People Properties based off of Events (e.g., Last app open or Number of app opens). So depending on your specific use case and what you’re trying to target, the best long-term solution would be to create a People Property that accomplishes the same goal.
An example might be targeting an In-App message to users who have completed a Login event within the last day. To accomplish this, set a People Property that tracks Last Login Date, and target an In-App message to all users whose Last Login Date was less than one day ago. Learn more about setting People Properties; learn more about sending a date as a property type.