A financial services firm is deploying a FortiGate-VM High Availability (HA) cluster in Azure using Terraform. To meet compliance requirements, all HA-related traffic, including FGCP heartbeat packets, must be isolated on a dedicated subnet. The lead architect has mandated that the Azure Load Balancer health probe must target a specific, non-standard port on the FortiGate's internal interface to monitor service health. Which Terraform resource and attribute is essential for configuring this custom health probe port?