Using the native Unity Package Manager introduced in 2017.2, you can add this library as a package by modifying your manifest.json file found at /ProjectName/Packages/manifest.json to include it as a dependency. See the example below on how to reference it.

The package is available on the npmjs registry.

{
  "dependencies": {
    ...
  },
  "scopedRegistries": [
    {
      "name": "Playdarium",
      "url": "https://registry.npmjs.org",
      "scopes": [
        "com.playdarium.unity"
      ]
    }
  ]
}

Open Window -> Package Manager choose Packages: My Regestries and install package

"com.playdarium.unity.simple-ui": "https://gitlab.com/pd-packages/simple-ui.git#upm"

public enum EWindowLayer
{
    Project,
    Local
}

public class WindowsInstaller : AWindowsInstaller
{
    [SerializedField] private EWindowLayer windowLayer;
    
	protected override string WindowLayer => windowLayer.ToString();
}