feat: add instructions for adding event api to backend

src/pages/[platform]/build-a-backend/data/connect-event-api/index.mdx

@@ -89,15 +89,179 @@ export default function App() {
     <>
       <button onClick={publishEvent}>Publish Event</button>
       <ul>
-        {myEvents.map((event, index) => (
-          <li key={index}>{JSON.stringify(event)}</li>
+        {myEvents.map((data) => (
+          <li key={data.id}>{JSON.stringify(data.event)}</li>
         ))}
       </ul>
     </>
   );
 }
 ```
 
-## Connect to an Event API with an existing Amplify backend
+## Add an Event API to an existing Amplify backend
 
-Coming Soon
+This guide walks through how you can add an Event API to an existing Amplify backend. We'll be using Cognito User Pools for authenticating with Event API from our frontend application. Any signed in user will be able to subscribe to the Event API and publish events.
+
+Before you begin, you will need:
+
+- An existing Amplify backend (see [Quickstart](/[platform]/start/quickstart/))
+- Latest versions of `@aws-amplify/backend` and `@aws-amplify/backend-cli` (`npm add @aws-amplify/backend@latest @aws-amplify/backend-cli@latest`)
+
+### Update Backend Definition
+
+First, we'll add a new Event API to our backend definition.
+
+```ts title="amplify/backend.ts"
+import { defineBackend } from '@aws-amplify/backend';
+import { auth } from './auth/resource';
+// highlight-start
+// import CDK resources:
+import {
+  CfnApi,
+  CfnChannelNamespace,
+  AuthorizationType,
+} from 'aws-cdk-lib/aws-appsync';
+import { Policy, PolicyStatement } from 'aws-cdk-lib/aws-iam';
+// highlight-end
+
+const backend = defineBackend({
+	auth,
+});
+
+// highlight-start
+// create a new stack for our Event API resources:
+const customResources = backend.createStack('custom-resources');
+
+// add a new Event API to the stack:
+const cfnEventAPI = new CfnApi(customResources, 'CfnEventAPI', {
+  name: 'my-event-api',
+  eventConfig: {
+    authProviders: [
+      {
+        authType: AuthorizationType.USER_POOL,
+        cognitoConfig: {
+          awsRegion: customResources.region,
+          // configure Event API to use the Cognito User Pool provisioned by Amplify:
+          userPoolId: backend.auth.resources.userPool.userPoolId,
+        },
+      },
+    ],
+    // configure the User Pool as the auth provider for Connect, Publish, and Subscribe operations:
+    connectionAuthModes: [{ authType: AuthorizationType.USER_POOL }],
+    defaultPublishAuthModes: [{ authType: AuthorizationType.USER_POOL }],
+    defaultSubscribeAuthModes: [{ authType: AuthorizationType.USER_POOL }],
+  },
+});
+
+// create a default namespace for our Event API:
+const namespace = new CfnChannelNamespace(
+  customResources,
+  'CfnEventAPINamespace',
+  {
+    apiId: cfnEventAPI.attrApiId,
+    name: 'default',
+  }
+);
+
+// attach a policy to the authenticated user role in our User Pool to grant access to the Event API:
+backend.auth.resources.authenticatedUserIamRole.attachInlinePolicy(
+  new Policy(customResources, 'AppSyncEventPolicy', {
+    statements: [
+      new PolicyStatement({
+        actions: [
+          'appsync:EventConnect',
+          'appsync:EventSubscribe',
+          'appsync:EventPublish',
+        ],
+        resources: [`${cfnEventAPI.attrApiArn}/*`, `${cfnEventAPI.attrApiArn}`],
+      }),
+    ],
+  })
+);
+
+// finally, add the Event API configuration to amplify_outputs:
+backend.addOutput({
+  custom: {
+    events: {
+      url: `https://${cfnEventAPI.getAtt('Dns.Http').toString()}/event`,
+      aws_region: customResources.region,
+      default_authorization_type: AuthorizationType.USER_POOL,
+    },
+  },
+});
+// highlight-end
+```
+
+### Deploy Backend
+
+To test your changes, deploy your Amplify Sandbox.
+
+```bash title="Terminal" showLineNumbers={false}
+npx ampx sandbox
+```
+
+### Connect your frontend application
+
+After the sandbox deploys, connect your frontend application to the Event API. We'll be using the [Amplify Authenticator component](https://ui.docs.amplify.aws/react/connected-components/authenticator) to sign in to our Cognito User Pool.
+
+If you don't already have the Authenticator installed, you can install it by running `npm add @aws-amplify/ui-react`.
+
+```tsx title="src/App.tsx"
+import { useEffect, useState } from 'react';
+import { Amplify } from 'aws-amplify';
+import { events, type EventsChannel } from 'aws-amplify/data';
+import { Authenticator } from '@aws-amplify/ui-react';
+import '@aws-amplify/ui-react/styles.css';
+import outputs from '../amplify_outputs.json';
+
+Amplify.configure(outputs);
+
+export default function App() {
+  const [myEvents, setMyEvents] = useState<Record<string, any>[]>([]);
+
+  useEffect(() => {
+    let channel: EventsChannel;
+
+    const connectAndSubscribe = async () => {
+      channel = await events.connect('default/channel');
+
+      channel.subscribe({
+        next: (data) => {
+          console.log('received', data);
+          setMyEvents((prev) => [data, ...prev]);
+        },
+        error: (err) => console.error('error', err),
+      });
+    };
+
+    connectAndSubscribe();
+
+    return () => channel && channel.close();
+  }, []);
+
+  async function publishEvent() {
+    await events.post('default/channel', { some: 'data' });
+  }
+
+  return (
+    <Authenticator>
+      {({ signOut, user }) => (
+        <>
+          <div>
+            <h1>Welcome, {user.username}</h1>
+            <button onClick={signOut}>Sign Out</button>
+          </div>
+          <div>
+            <button onClick={publishEvent}>Publish Event</button>
+            <ul>
+            {myEvents.map((data) => (
+              <li key={data.id}>{JSON.stringify(data.event)}</li>
+              ))}
+            </ul>
+          </div>
+        </>
+      )}
+    </Authenticator>
+  );
+}
+```