var email = { "ID": "someID", "Body": "whatever" };
                var json = '{tmpEmail:' + JSON.stringify(email) + '}';
 $.ajax({
                    type: "POST",
                    url: "send.aspx/SendMessage",
                    contentType: "application/json; charset=utf-8",
                    data: json,
                    dataType: "text",
                    success: function (output) {
                        alert('Message Sent')
                    },
                    error: function (output) {
                        alert('error');
                    }
                });

<WebMethod> _
<ScriptMethod(ResponseFormat:=ResponseFormat.Json)> _
    Public Shared Async Function SendMessage(tmpEmail As tmpEmail) As Threading.Tasks.Task(Of String)
    Dim _EmailID As Guid
    _EmailID = Await EmailFunctions.SendEmail(tmpEmail)
    Return "Hey it works"
End Function

Public Shared Async Function SendEmailLead(obj as tmpEmail) As Task(Of Guid)
 Dim PClient As New PostmarkDotNet.PostmarkClient(APIKey)
    Dim Result As PostmarkDotNet.PostmarkResponse = await PClient.SendMessageAsync(obj)
    If Result.Status = PostmarkDotNet.PostmarkStatus.Success Then
       ...
        Return LogID
    Else
        ... throw some exception
    End If
End Function