To create an Xbox background wallpaper using the Xbox Mastercard, you can use a tool like Adobe Illustrator or Inkscape to design the vector image.

Here's a brief snippet in SVG format for the Xbox Mastercard:

<svg width="100%" height="100%" viewBox="0 0 512 512" xmlns="http://www.w3.org/2000/svg">
    <!-- Add your design elements here -->
    <rect x="10" y="10" width="490" height="490" fill="#000000" rx="20"/>
    <path d="M 256, 256 L 256, 128 L 288, 128 L 288, 256 L 256, 256 Z"/>
</svg>

You can customize this SVG code to fit your design needs. Save it as an image file (e.g., PNG or JPEG) and set it as your Xbox background wallpaper.

Note: This is a basic example, and you may need to adjust the dimensions, colors, and shapes to match the desired look of the Xbox Mastercard.