diff --git a/terraform/application/application.tf b/terraform/application/application.tf
index 681d651efe..79b4cbc918 100644
--- a/terraform/application/application.tf
+++ b/terraform/application/application.tf
@@ -15,7 +15,8 @@ module "application_configuration" {
     {
       ENVIRONMENT_NAME = var.environment
       PGSSLMODE        = local.postgres_ssl_mode
-  })
+      CANONICAL_HOSTNAME = local.canonical_hostname
+    })
   secret_variables = {
     DATABASE_URL = module.postgres.url
   }
diff --git a/terraform/application/config/production.tfvars.json b/terraform/application/config/production.tfvars.json
index 25bc897d9a..c91ef09893 100644
--- a/terraform/application/config/production.tfvars.json
+++ b/terraform/application/config/production.tfvars.json
@@ -2,6 +2,7 @@
   "cluster": "production",
   "namespace": "srtl-production",
   "environment": "production",
+  "canonical_hostname": "claim-additional-payments-for-teaching-production.teacherservices.cloud",
   "enable_postgres_backup_storage": true,
   "enable_monitoring": true,
   "external_url": "https://claim-additional-teaching-payment.service.gov.uk/healthcheck",
diff --git a/terraform/application/config/test.tfvars.json b/terraform/application/config/test.tfvars.json
index b64515add3..3257f5f887 100644
--- a/terraform/application/config/test.tfvars.json
+++ b/terraform/application/config/test.tfvars.json
@@ -1,5 +1,6 @@
 {
   "cluster": "test",
   "namespace": "srtl-test",
-  "environment": "test"
+  "environment": "test",
+  "canonical_hostname": "claim-additional-payments-for-teaching-test.test.teacherservices.cloud"
 }
diff --git a/terraform/application/variables.tf b/terraform/application/variables.tf
index f602f3b136..d4d0bfa9e1 100644
--- a/terraform/application/variables.tf
+++ b/terraform/application/variables.tf
@@ -53,8 +53,15 @@ variable "config" {
   type = string
 }
 
+variable "canonical_hostname" {
+  type        = string
+  description = "External domain name for the app service"
+  default     = null
+}
+
 locals {
   postgres_ssl_mode = var.enable_postgres_ssl ? "require" : "disable"
+  canonical_hostname = var.canonical_hostname != null ? var.canonical_hostname : "${var.service_name}-${var.environment}.test.teacherservices.cloud"
   app_env_values_from_yml = yamldecode(file("${path.module}/config/${var.config}_app_env.yml"))
   app_env_values = merge(local.app_env_values_from_yml)
 }