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--- title: Include file description: Include file services: azure-communication-services author: shamkh ms.service: azure-communication-services ms.subservice: advanced-messaging ms.date: 05/01/2025 ms.topic: include ms.custom: include file ms.author: shamkh ms.custom: devx-track-extended-java, devx-track-js, devx-track-python zone_pivot_groups: acs-js-csharp-java-python --- ### Authenticate the client The Messages SDK uses the `NotificationMessagesClient` to send messages. The `NotificationMessagesClient` method authenticates using your connection string acquired from Azure Communication Services resource in the Azure portal. For more information about connection strings, see [access-your-connection-strings-and-service-endpoints](../../../create-communication-resource.md#access-your-connection-strings-and-service-endpoints). [!INCLUDE [Authenticate the client](./authenticate-notification-messages-client-net.md)] ### Set channel registration ID You created the Channel Registration ID GUID during [channel registration](../connect-whatsapp-business-account.md). Find it in the portal on the **Channels** tab of your Azure Communication Services resource. :::image type="content" source="../media/get-started/get-messages-channel-id.png" lightbox="../media/get-started/get-messages-channel-id.png" alt-text="Screenshot that shows an Azure Communication Services resource in the Azure portal, viewing the 'Channels' tab. Attention is placed on the copy action of the 'Channel ID' field."::: Assign it to a variable called channelRegistrationId. ```csharp var channelRegistrationId = new Guid("<your channel registration ID GUID>"); ``` ### Set recipient list You need to supply an active phone number associated with a WhatsApp account, or a business-scoped user ID (BSUID). This WhatsApp account receives the template, text, and media messages sent in this quickstart. For this example, you can use your personal phone number. The recipient phone number can't be the business phone number (Sender ID) associated with the WhatsApp channel registration. The Sender ID appears as the sender of the text and media messages sent to the recipient. The phone number must include the country code. For more information about phone number formatting, see WhatsApp documentation for [Phone Number Formats](https://developers.facebook.com/docs/whatsapp/cloud-api/reference/phone-numbers#phone-number-formats). > [!NOTE] > Only one phone number or BSUID is currently supported in the recipient list. Create the recipient list like this: ```csharp var recipientList = new List<string> { "<to WhatsApp phone number or BSUID>" }; ``` Example using a phone number: ```csharp // Example only var recipientList = new List<string> { "+14255550199" }; ``` Example using a BSUID: ```csharp // Example only var recipientList = new List<string> { "US.13491208655302741918" }; ``` > [!NOTE] > Sending messages to BSUIDs will be available starting in June 2026. Until then, use phone numbers as recipients. For more information about BSUIDs, see [WhatsApp usernames and BSUIDs](../../../../concepts/advanced-messaging/whatsapp/whatsapp-username-support-overview.md). ### Start sending messages between a business and a WhatsApp user Conversations between a WhatsApp Business Account and a WhatsApp user can be initiated in one of two ways: - The business sends a template message to the WhatsApp user. - The WhatsApp user sends any message to the business number. A business can't initiate an interactive conversation. A business can only send an interactive message after receiving a message from the user. The business can only send interactive messages to the user during the active conversation. Once the 24 hour conversation window expires, only the user can restart the interactive conversation. For more information about conversations, see the definition at [WhatsApp Business Platform](https://developers.facebook.com/docs/whatsapp/pricing#conversations). To initiate an interactive conversation from your personal WhatsApp account, send a message to your business number (Sender ID). :::image type="content" source="../media/get-started/user-initiated-conversation.png" lightbox="../media/get-started/user-initiated-conversation.png" alt-text="A WhatsApp conversation viewed on the web showing a user message sent to the WhatsApp Business Account number.":::