HttpClient timeout error calling PostAsJsonAsync

I am having a problem getting the response from the HttpClient PostAsJsonAsync method. I am passing a StringContent data to a POST request but it returns a Forbidden (403) status code.

When I tried adding a default request header (commented line of code), the error changed and it returned a timeout issue:

{System.Net.Http.WinHttpException (0x80072EE2): The operation timed out at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Threading.Tasks.RendezvousAwaitable`1.GetResult() at System.Net.Http.WinHttpHandler.d__105.MoveNext()}

var content = new StringContent(JsonConvert.SerializeObject(contentBody), Encoding.UTF8, "application/json");

var client = new HttpClient();
client.BaseAddress = new Uri("https://www.example.com");
//client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

using (var httpResponse = await client.PostAsJsonAsync("api/details", content))
{
    if (httpResponse.Content != null)
    {
        var responseContent = await httpResponse.Content.ReadAsStringAsync();
    }
}

When I tried it in Postman, it returned the JSON data. Why doesn't it work in the code? Is there something that blocks my connection?

The BaseAddress property needs to be suffixed with a forward slash:

client.BaseAddress = new Uri("https://www.example.com/");