Hello @Ernesto
Currently, Front door does not support Webapp slots as Backend pools can be composed of Storage, Web App, Kubernetes instances, or any other custom hostname that has public connectivity.
As Azure Front Door requires that the backends are defined either via a public IP or a publicly resolvable DNS hostname. Members of backend pools can be across zones, regions, or even outside of Azure as long as they have public connectivity. So you can try to add your slot DNS name.
https://learn.microsoft.com/en-us/azure/frontdoor/front-door-faq#what-resources-are-supported-today-as-part-of-backend-pool-
https://stackoverflow.com/questions/66102743/azure-app-service-deployment-slot-application-gateway