diff --git a/packages/backend-auth/src/lambda/reference_auth_initializer.ts b/packages/backend-auth/src/lambda/reference_auth_initializer.ts
index 29e9aa18c3a..f25e0c4ec0a 100644
--- a/packages/backend-auth/src/lambda/reference_auth_initializer.ts
+++ b/packages/backend-auth/src/lambda/reference_auth_initializer.ts
@@ -459,7 +459,9 @@ export class ReferenceAuthInitializer {
 
     // domain
     const oauthDomain = userPool.CustomDomain ?? userPool.Domain ?? '';
-    const fullDomainPath = `${oauthDomain}.auth.${region}.amazoncognito.com`;
+    const fullDomainPath = userPool.CustomDomain
+      ? userPool.CustomDomain
+      : `${oauthDomain}.auth.${region}.amazoncognito.com`;
     const data = {
       signupAttributes: JSON.stringify(
         userPool.SchemaAttributes?.filter(