VB.netでTeamsに個人宛のチャットを送りたいのだがうまく行かない

以下のソースコードでやってみたのですがうまく行きません。。。

情報も少なく困っておりますうまく行った事例などあればご教授願います

Private Const clientId As String = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

Private Const tenantId As String = "zzzzzzzzzzzzzzzzzzzzzzzzzzz"

Private Const clientSecret As String = "yyyyyyyyyyyyyyyyyyyyyyyy"

Private Const GraphApiEndpoint As String = "https://graph.microsoft.com/v1.0"

Private Async Sub button3_Click(sender As Object, e As EventArgs) Handles Button3.Click

    Try

        Dim accessToken = Await GetAccessTokenAsync()

        Dim messageContent = "テストメッセージ！"

        Dim jsonBody = $"{{""body"": {{""content"": ""{messageContent}"", ""contentType"": ""html""}}}}"

        Dim recipientUserId = "KKKK@DDD.DD" 

        Dim otherServiceEndpoint = "chats" 

        Dim requestUrl = $"{GraphApiEndpoint}/users/{recipientUserId}/{otherServiceEndpoint}"

        Dim httpClient = New HttpClient()

        Dim request = New HttpRequestMessage(HttpMethod.Post, requestUrl)

        request.Headers.Authorization = New System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", accessToken)

        request.Content = New StringContent(jsonBody, Encoding.UTF8, "application/json")

 

        Dim response = Await httpClient.SendAsync(request)

        ' 応答の処理

        If response.IsSuccessStatusCode Then

            Dim responseBody = Await response.Content.ReadAsStringAsync()

            TextBox1.Text = $"メッセージの送信に成功しました。レスポンス: {responseBody}"

        Else

            Dim errorMessage = $"エラー: {response.StatusCode} - {response.ReasonPhrase}"

            TextBox1.Text = errorMessage

        End If

    Catch ex As Exception

        TextBox1.Text = $"エラー: {ex.Message}"

    End Try

End Sub

Private Async Function GetAccessTokenAsync() As Task(Of String)

    Dim confidentialClient = ConfidentialClientApplicationBuilder _

        .Create(clientId) _

        .WithTenantId(tenantId) _

        .WithClientSecret(clientSecret) _

        .Build()

    Dim authResult = Await confidentialClient.AcquireTokenForClient({"https://graph.microsoft.com/.default"}).ExecuteAsync()

    Return authResult.AccessToken

End Function