OK, let me explain what you need to do. At the moment, CFD apps are .NET Core 1.0 DLLs. In the near future we will update the CFD to produce .NET Core 2.0 DLLs. This is like this because until 3CX v15.5 SP2, 3CX was running on .NET Core 1.0, and now it has been upgraded, so we need to also update the CFD. Until we do this, the apps are .NET Core 1.0 and are loaded by 3CX without any problem as they are compatible.
So, in order to work with the CFD, you need to use .NET Core 1.0 DLL references, and then in 3CX the version 2.0 will be used. As a result, you need to download from Nuget the DLL for .NET Core 1.0 and copy it to the CompilerDependencies folder into the 3CX CFD installation folder. This way, the DLL is used to build the app, and in runtime the DLL 2.0 will be taken from the 3CX installation folder.
In order to get the right DLL from Nuget, you just need to create a project with Visual Studio adding a reference to the library you need. Then nuget will download all the versions of the DLL and you can copy the version you need from there. For example, nuget will download the DLLs to the following folder:
C:\Users\USER_NAME\.nuget\packages\PACKAGE_NAME\PACKAGE_VERSION\lib\netstandard1.6
You need to get the DLL for netstandard1.6 or netstandard1.3, check which folder you have.
Kind regards.