Solution [program .csproj]:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFrameworks>net481;net9.0-windows</TargetFrameworks> 
    <UseWindowsForms>true</UseWindowsForms>
    <LangVersion>latest</LangVersion>
  </PropertyGroup>
  <ItemGroup>
    <ProjectReference Include="..\TestProjLib\TestProjLib.csproj" />
  </ItemGroup>
</Project>

... This theemes to work, more tests to do but for now I am very thankful.