Broken back glass on my iphone XS Max

You can schedule an appointment at your Apple Store Genius Bar. You can use the Contact Support button at the top of this page. See this link which provides more information, including how much it will cost for a replacement phone --> iPhone Repair - Official Apple Support

Since the device must be replaced and accidental damage is not covered by the standard warranty the cost is yours.

Use the Get Support link (top of this page) to find out how much.