I haven't been able to try this until today. Unfortunately rbwhitaker's 503, so I can't check if the extra steps I had to do, but those are here just in case somebody needs them also (like myself and my fish-like memory )
The project was very very old (I created it back in Jan'2014), it probably started as a Xamarin Studio project, so newer projects might not need this.
In order to get the project compiling and working I also had to add this to the .csproj file:
<Import Project="$(MSBuildExtensionsPath)\MonoGame\v3.0\MonoGame.Content.Builder.targets" />
And edit mgcb file to build as MonoGameContentReference.
Then MGCB was complaining about the platform, so added in the first PropertyGroup: