Using System.Windows.Forms classes in a .net core 3.0 preview9 project

You should add <UseWindowsForms>true</UseWindowsForms> in your csproj.

<PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>netcoreapp3.0</TargetFramework>
    <UseWpf>true</UseWpf>
    <UseWindowsForms>true</UseWindowsForms>
</PropertyGroup>