Suggested project

MonoGameProject.xml

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFrameworks>net45;netcoreapp3.0</TargetFrameworks>
  </PropertyGroup>

  <PropertyGroup Condition=" '$(TargetFramework)' == 'net45'">
    <DefineConstants>$(DefineConstants);NETFULL</DefineConstants>
  </PropertyGroup>

  <PropertyGroup Condition=" '$(TargetFramework)' == 'netcoreapp3.0'">
    <DefineConstants>$(DefineConstants);NETCORE</DefineConstants>
  </PropertyGroup>

  <ItemGroup Condition="'$(TargetFramework)' == 'net45'">
    <Reference Include="System.Windows.Forms" />
  </ItemGroup>

  <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0'">
    <Reference Include="System.Windows.Forms">
      <HintPath>C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App\3.0.0\System.Windows.Forms.dll</HintPath>
    </Reference>
    <Reference Include="System.Drawing.Common">
      <HintPath>C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App\3.0.0\System.Drawing.Common.dll</HintPath>
    </Reference>
  </ItemGroup>
  
</Project>