Try something like:
Private Async Sub PostMessageTest_Click(sender As Object, e As EventArgs) Handles PostMessageTest.Click
Dim c As New CustomersRest
c.FirstName = "Bill"
c.LastName = "Gates"
c.CustomerID = Guid.Empty
Dim RestURL As String = "https://example.com/api/customers/"
Dim client As New Http.HttpClient
Dim JsonData As String = JsonConvert.SerializeObject(c)
Dim RestContent As New Http.StringContent(JsonData, Encoding.UTF8, "application/json")
Dim RestResponse As Http.HttpResponseMessage = Await client.PostAsync(RestURL, RestContent)
ResultMessage.Text = RestResponse.StatusCode.ToString
End Sub
Data passed in body is of type
"application/json"
Here, for reference:
Setup POST REST Webservice with ASP.net and VB.net - dotnetco.de[
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How to: Send data by using the WebRequest class | Microsoft Docs[
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