Override each Native HttpClientHandler to disable the HTTPS Certificate Validation of the endpoints that we access from our App.

XamarinNativeDisableHttpsCertValidation.cs

iOS:

// override HTTPS Certificate Verification iOS

// In AppDelegate.cs 

var iosClientHandler = new NSUrlSessionHandler();
iosClientHandler.TrustOverride += (sender, trust) =>
{
	return true;
};
Services.HttpClientService.HttpClientHandler = iosClientHandler;

--------------------------------------------------------------------------------------

Android:

// override HTTPS Certificate Verification Android 

/// <summary>
/// Custom AndroidClientHandler for disabling
/// Self-Signed HTTPS Certificate Validation
/// Credits: https://nicksnettravels.builttoroam.com/android-certificates/
/// </summary>
public class CustomAndroidClientHandler : AndroidClientHandler
{
	protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
	{
		request.Version = new System.Version(2, 0);
		return await base.SendAsync(request, cancellationToken);
	}

	protected override SSLSocketFactory ConfigureCustomSSLSocketFactory(HttpsURLConnection connection)
	{
		return SSLCertificateSocketFactory.GetInsecure(0, null);
	}

	protected override IHostnameVerifier GetSSLHostnameVerifier(HttpsURLConnection connection)
	{
		return new BypassHostnameVerifier();
	}
}

internal class BypassHostnameVerifier : Java.Lang.Object, IHostnameVerifier
{
	public bool Verify(string hostname, ISSLSession session)
	{
		return true;
	}
}

...
...

// In MainActivity.cs

protected override void OnCreate(Bundle savedInstanceState)
{
...
	var androidClientHandler = new CustomAndroidClientHandler();
	Services.HttpClientService.HttpClientHandler = 
                                          androidClientHandler;
...
}

--------------------------------------------------------------------------------------

Windows (UWP):

// override HTTPS Certificate Verification Android 

// In MainPage.xaml.cs 

var uwpClientHandler = new WinHttpHandler()
{
	ServerCertificateValidationCallback =
	   (message, certificate2, arg3, arg4) =>
	   {
		   return true;
	   }
};

Services.HttpClientService.HttpClientHandler = uwpClientHandler;