+1 for working directly with source, you can integrate your extension to the MonoGame processor app like a breeze if you go that way, no hassle involved*.
(* additional hassle might be involved, please consult your local software engineer)
Thanks @mgarstenauer , you were right, I was looking at C:\Program Files (x86)\MonoGame\v3.0\Assemblies but I could find the DLL where you said, thanks
I will consider build from source… thanks for the tip @KraHen