Use the supplied token in the Custom User sign in

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

.changeset/rich-fireants-happen.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core-components': patch
+---
+
+Ensure that the "Custom User" sign-in makes use of supplied custom tokens, if any

packages/core-components/src/layout/SignInPage/customProvider.tsx

@@ -69,10 +69,11 @@ const Component: ProviderComponent = ({ onSignInSuccess }) => {
 
   const { errors } = formState;
 
-  const handleResult = ({ userId }: Data) => {
+  const handleResult = ({ userId, idToken }: Data) => {
     onSignInSuccess(
       UserIdentity.fromLegacy({
         userId,
+        getIdToken: idToken !== undefined ? async () => idToken : undefined,
         profile: {
           email: `${userId}@example.com`,
         },