This repo contains a standalone test application that uses a Direct3D12 mesh shader. Good to use as a starting point or for verifying basic things. It was based off of the spinning cube sample code, see here.
1 (numerical key) - Input assembler mode
2 (numerical key) - Mesh shader mode
The current mode is written in the application title bar. If mesh shaders are not supported on the device, it says "Input Assembler only, Mesh Shaders not supported on device" in the title bar, and the program will use input assembler only.