HRESULT XGameUiShowWebAuthenticationAsync(  
         XAsyncBlock* async,  
         XUserHandle requestingUser,  
         const char* requestUri,  
         const char* completionUri  
)  

// Use Facebook example for OAuth; client_id should correspond to your registered application.
const char completionUri[] = "https://www.facebook.com/connect/login_success.html"; 
const char requestUri[] =
    "https://www.facebook.com/dialog/oauth?"
    "client_id=000000000000000&"
    "redirect_uri=https%3A%2F%2Fwww.facebook.com%2Fconnect%2Flogin_success.html&"
    "response_type=token&"
    "sdk=xboxone";

// Allocate and initialize XAsyncBlock for asynchronous authentication operation.
XAsyncBlock* block = new XAsyncBlock();
ZeroMemory(block, sizeof(XAsyncBlock));
block->callback = [](XAsyncBlock* block)
{
    // Query required size and allocate buffer for authentication result data.
    uint32_t bufferSize = 0;
    FAIL_FAST_IF_FAILED(XGameUiShowWebAuthenticationResultSize(block, &bufferSize));
    uint8_t* buffer = new uint8_t[bufferSize];

    // The currentUser is initialized with user to authenticate for.
    XGameUiWebAuthenticationResultData* resultData = nullptr;
    FAIL_FAST_IF_FAILED(XGameUiShowWebAuthenticationResult(
        block,
        bufferSize,
        buffer,
        &resultData,
        nullptr
        ));

    //
    // Use the result data here. If resultData->responseStatus is S_OK, then web authentication was successful.
    //

    // Free allocated buffer and XAsyncBlock.
    delete[] buffer;
    delete block;
};

//  Begin asynchronous authentication operation.
XUserHandle currentUser = GetCurrentUserForAuthentication();
FAIL_FAST_IF_FAILED(XGameUiShowWebAuthenticationAsync(
    block,
    currentUser,
    requestUri,
    completionUri
    ));