Error passing JSON to API

The problem might be due to the way you’re serializing your data. When you manually create a JSON string with double quotes ("{""contractor"":{""numero"":""015380"",""tipo"":2}"), it works fine. However, when you use JsonSerializer.Serialize, which uses single quotes, it doesn’t work.

Try with single quotes.

        Dim NovasMensagens As New EntradaIndicadorNovasMensagens()

        NovasMensagens.contratante = New Contratante() With {
            .numero = 123456789,
            .tipo = 2
        }

        Dim jsonString As String = JsonSerializer.Serialize(NovasMensagens)

        Using httpClient = New HttpClient()
            Using request As New HttpRequestMessage(HttpMethod.Post, URLBASE & "Monitorar")
                request.Headers.TryAddWithoutValidation("accept", "*/*")
                request.Headers.TryAddWithoutValidation("jwt_token", JWK_Token)
                request.Headers.TryAddWithoutValidation("Authorization", "Bearer " & Acesss_Token)
                request.Headers.TryAddWithoutValidation("Content-Type", "application/json")

                Dim stringContent As New StringContent(jsonString, Encoding.UTF8, "application/json")

                Dim response As Task(Of HttpResponseMessage) = httpClient.SendAsync(request)

                Return Await response.Result.Content.ReadFromJsonAsync(Of RetornoIndicadorNovasMensagens)(OpcoesDesserializarJson)

            End Using
        End Using