I was able to get this working! The thing that's most important to know is that you CANNOT just import the .cer file from your developer portal into your Windows cert store and start attaching it to your requests. In order to get it to work, I had to: Create a .pfx file (which will, by nature, contain both the cert and its key) as shown here: https://blog.aclerbois.be/blog/2020/01/09/Generates-a-pfx-for-Apple-Pay Import the generated .pfx file into my Windows cert store. Attach the cert to my WebRequestHandler Here's my C# code. Sorry, you'll have to translate it to VB: public async Task<ApplePaySession> GetSessionAsync(string validationUrl = null) { var sessionRequestBody = new ApplePaySessionRequest { merchantIdentifier = ConfigurationManager.AppSettings["ApplePayMerchantIdentifier"], displayName = "DISPLAY NAME", initiativeContext = ConfigurationManager.AppSettings["ApplePayInitiativeUrl"] }; var handler = new WebRequestHandler(); handler.ClientCertificates.Add(GetCertificate(MerchantIdentityCertThumbprint)); var client = new HttpClient(handler); var url = !string.IsNullOrWhiteSpace(validationUrl) ? validationUrl : PaymentSessionEndpointUrl; var response = await client.PostAsJsonAsync(url, sessionRequestBody); var applePaySession = await response.Content.ReadAsAsync<ApplePaySession>(); return applePaySession; } private X509Certificate2 GetCertificate(string certThumbprint) { X509Store userCaStore = new X509Store(StoreName.My, StoreLocation.CurrentUser); try { userCaStore.Open(OpenFlags.ReadOnly); X509Certificate2Collection certificatesInStore = userCaStore.Certificates; X509Certificate2Collection findResult = certificatesInStore.Find(X509FindType.FindByThumbprint, certThumbprint, false); X509Certificate2 clientCertificate = null; if (findResult.Count == 1) { clientCertificate = findResult[0]; } else { throw new Exception("Unable to locate the correct client certificate."); } return clientCertificate; } catch { throw; } finally { userCaStore.Close(); } } I hope this helps! I spent a long time piecing together random posts to get this one done.