There is no sample code for your problem. The message is quite clear you cannot mix managed and unmanaged code in the same project this way. You need to split it into two projects, one which is normal unmanaged C++ and one which is managed and uses the /clr option. Calling Managed Code from Unmanaged Code and vice-versa[^] is a CodeProject article that will help you.
One of these days I'm going to think of a really clever signature.