"Exclusive Trust" Certificate Pinning on Windows 8.1 Phone App - ssl

Question

So I'm trying to develop a secure Windows 8.1 Phone App. Note that this is for use on Windows Phone, and not on normal Windows. Of main concern here are Man-In-The-Middle (MITM) attacks.
The app was generated using Microsoft App Studio (http://appstudio.windows.com/) and edited using Visual Studio 2015.
I can pin a certificate to the normal Windows section using the instructions on this MS blogpost: https://blogs.msdn.microsoft.com/wsdevsol/2014/06/05/including-self-signed-certificates-with-your-windows-runtime-based-windows-phone-8-1-apps/ but the problem is that the user can upload a certificate to the device which will override the check from this app.
The problem with this approach is that the Package.appxmanifest interface for the Windows Phone App doesn't allow you to update a certificate, let alone mark it as 'exclusive'.
My second attempt was to hardcode the Fingerprint of the Server certificate as per this post: https://blogs.windows.com/buildingapps/2015/10/13/create-more-secure-apps-with-less-effort-10-by-10/ (Note the checkbox for "Exclusive Trust")
Although the post is for Windows 10, the code worked (after adding in the proper imports). However, again, when one updates their own certificate, it bypasses the fingerprint check.
Because I can't upload the certificate to the Phone App part using the Visual Studio Interface, and therefore can't check the box for "Exclusive Trust", how would I go about making sure that any request to the website only uses the certificate I upload or the Fingerprint I hardcode?
Here's the fingerprinting code:
using System;
using System.Net;
using Windows.Web.Http;
using System.Collections.Generic;
using Windows.System;
using Windows.Security.Cryptography.Certificates;
using The_App.Views;
namespace The_App.Services
{
public class NavigationServices
{
static public void NavigateToPage(string pageName, object parameter = null)
{
try
{
string pageTypeName = String.Format("{0}.{1}", typeof(MainPage).Namespace, pageName);
Type pageType = Type.GetType(pageTypeName);
App.RootFrame.Navigate(pageType, parameter);
}
catch (Exception ex)
{
AppLogs.WriteError("NavigationServices.NavigateToPage", ex);
}
}
static public async void NavigateTo(Uri uri)
{
try
{
// Send a get request to uri
HttpClient client = new HttpClient();
HttpResponseMessage response = await client.GetAsync(uri);
// Get the list of certificates that were used to validate the server's identity
IReadOnlyList<Certificate> serverCertificates = response.RequestMessage.TransportInformation.ServerIntermediateCertificates;
// Perform validation
if (!ValidCertificates(serverCertificates))
{
// Close connection as chain is not valid
return;
}
// PrintResults("Validation passed\n");
// Validation passed, continue with connection to service
await Launcher.LaunchUriAsync(uri);
}
catch (Exception ex)
{
AppLogs.WriteError("NavigationServices.NavigateTo", ex);
}
}
private static bool ValidCertificates(IReadOnlyList<Certificate> certs)
{
// In this example, we iterate through the certificates and check that the chain contains
// one specific certificate we are expecting
for (int i = 0; i < certs.Count; i++)
{
// PrintResults("Cert# " + i + ": " + certs[i].Subject + "\n");
byte[] thumbprint = certs[i].GetHashValue();
// Check if the thumbprint matches whatever you are expecting
// <The SHA1 Server Certificate HEX Number in the comments>
byte[] expected = new byte[] { <The SHA1 Server Certificate Number in Bytes 45, 15, etc.> };
if (thumbprint == expected)
{
return true;
}
}
return false;
}
}
}