Cannot add shaders to "Shader Preloading"

I can add shaders to "Always Included Shaders", but it won't let me add anything to "Shader Preloading"...I'm getting some lag spikes at the end of my loading screens, and I believe it's caused by shaders loading. Any idea why it won't let me add any? I've tried dragging and dropping as well as clicking the little circle icon, but an empty window pops up and it won't let me search for any.

 

"Shader preloading" is for ShaderVariantCollections, not Shaders themselves. Just right-click in your Assets folder and select "Create -> Shader -> Shader Variant Collection". You can then select the new asset and add shaders to the collection, and use that collection for Shader Preloading.