Apple Pro Display XDR to connect with MacPro 2013?

Genuine ThunderBolt displays can be supported using the Apple ThunderBolt-2 <-> Thunderbolt-3 adapter and a genuine Thunderbolt cable on the appropriate side.

The late 2013 Mac Pro supports up to 5K displays, short of the top resolution of the HDR 6K display. so it will display a picture, but at lower resolution than you paid for.