I am sorry, but the question is still very unclear... You don't have to share your project, but at least give us more informations. For now I'll try to interpret your question as best as I can, and I'll edit it later if you explain yourself better.
The way I understood it is that you want your light to shine through the top face of your object, but not through the bottom faces. I assume you want your top face to be a solid material, and I assume you want to keep it all as one single object. I'll also assume the spotlight direction is always above the object and that you don't want to have extra objects (e.g. shadow catchers). As you can see a lot of assumptions here.
In this case you can do it with 2 different materials applied to the cube. On the bottom faces you have your normal material, while on the top face you have this material:
The principled shader is your material, the other nodes are just there to allow the light to go through the face itself