I was looking into this early in my project and from what I read, there’s no support because MonoGame uses MojoShader which doesn’t support Geometry Shaders. There have been posts here and there abou adding support but it looks it opens up issues because of mobile support so I don’t think they ever really went anywhere.
Looks like Jjagg might have been looking into some support for this a while back. Unfortunately it was GLSL and not HLSL, but might be worth asking if he ever got it working.