HTTPS request fails using HttpClient

This was a very helpful document. For ASP.NET Core 2.0, the answer was applied as follows (the result was successful):

using (var handler = new HttpClientHandler())
{
    handler.ServerCertificateCustomValidationCallback = (sender, certificate, chain, sslPolicyErrors) => true;
    handler.SslProtocols = SslProtocols.Tls12 | SslProtocols.Tls11 | SslProtocols.Tls;
    using (HttpClient client = new HttpClient(handler))
    {
        string requestObjJson = requestObj.ToJson();
        var address = new Uri($"https://yourcompany.com/");
        string token = GetToken();
        client.BaseAddress = address;
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
        var contentData = new StringContent(requestObjJson, System.Text.Encoding.UTF8, "application/json");
        using (var response = await client.PostAsync("yourcompany/new-employee", contentData))
        {
            var content = response.Content.ReadAsStringAsync();
            var taskResult = content.Result;
            JObject resultObj = JObject.Parse(taskResult);
            return resultObj;
        }
    }
}

According to this SO post, you must enable TLS1.2 with ServicePointManager.

System.Net.ServicePointManager.SecurityProtocol |=
    SecurityProtocolType.Tls12 | 
    SecurityProtocolType.Tls11 | 
    SecurityProtocolType.Tls; // comparable to modern browsers

Also noteworthy, the MSDN documentation for ServicePointManager.SecurityProtocols property makes this statement:

The .NET Framework 4.6 includes a new security feature that blocks insecure cipher and hashing algorithms for connections.

which suggests that some form of SHA1 block might be in place.

EDIT 16 Sep 2020

I changed from the = assignment operator to the |= operator so that requests to any other legacy sites which still require SSL will continue to work.