You'll need to book a tour through a travel agency in Tibet or China

The Tibet Visa Permit is issued by the Tibet Tourism Bureau (TTB) and is required for all non-Chinese travelers visiting Tibet. Here's how to obtain it:In most cases, you need to book a tour with a registered travel agency in Tibet or China that specializes in Tibetan travel. They will arrange your itinerary and apply for the Tibet Travel Permit on your behalf.