Unfortunately, until now the Terraform still does not support this point clearly because Azure APIs aren't enough yet. But in my case, I want to route traffic to many back end pool as each function app. I try to pass default domain as fqdns. When Terraform runs completely, I recognize the Azure Gateway implicit know it's type of app service plan and adjust the type in the portal and work correctly as I expect.

backend_address_pool {
  name  = "<BACKEND_ADDRESS_POOL_NAME>"
  fqdns = ["<DEFAULT_DOMAIN_NAME_FUNCTION_APP>"]
}