Set properties.webApplicationFirewallPolicyLink attribute within the id of firewall policy.

Set web_application_firewall_policy_link_id attribute within the frontend_endpoint block.

Example Configuration

resource "azurerm_frontdoor" "allowed" {
  name                                         = "example-frontdoor562"
  resource_group_name                          = azurerm_resource_group.example562.name

  routing_rule {
    name               = "exampleRoutingRule1"
    accepted_protocols = ["Http", "Https"]
    patterns_to_match  = ["/*"]
    frontend_endpoints = ["examplefrontend562"]
    
    forwarding_configuration {
      forwarding_protocol = "MatchRequest"
      backend_pool_name   = "exampleBackendBing"
    }
  }

  backend_pool_load_balancing {
    name = "exampleLoadBalancingSettings1"
  }

  backend_pool_health_probe {
    name = "exampleHealthProbeSetting1"
  }

  backend_pool {
    name = "exampleBackendBing"
    backend {
      host_header = "www.bing.com"
      address     = "www.bing.com"
      http_port   = 80
      https_port  = 443
    }

    load_balancing_name = "exampleLoadBalancingSettings1"
    health_probe_name   = "exampleHealthProbeSetting1"
  }

  frontend_endpoint {
    name                                    = "examplefrontend562"
    host_name                               = "example-frontdoor562.azurefd.net"
    web_application_firewall_policy_link_id = azurerm_frontdoor_firewall_policy.example562.id
  }
}