How to create new cube on click of a mouse?

I've been away from gd for awhile so i'm a bit rusty on giving help, but generally the process works better when you have a specific problem that can be shown with code. I'm seeing that you want to create a cube on key press. what does your WM_KEYDOWN handler look like in the windows message procedure? Do you understand the process of creating buffers? if you want a single model(cube) drawn multiple times you will need some method of storing the transforms for each object. then in the render function you setup the transform and render the geometry for each object in a loop.

// c++ psuedo code

// ...in WM_KEYDOWN HANDLER
if(key == b)
{
  sceneobject o;
  // we create the buffers somewhere else maybe on level load, or game start, but they exist already
  o.vertbuffer = &cubebuffer
  o.indexbuffer = &cubeindices
  // world matrix that will be a randon position between 0..5
  o.transform = Matrix4Position(rand % 10 - 5, rand % 10 - 5, rand % 10 - 5) 
  
  sceneobjects.push_back(o)
}
// END WM_KEYDOWN HANDLER
    
// ...somewhere in render function

//begin scene

for(int modelindex =0; modelindex < sceneobjects.size(); modelindex++)
{
  // setup transforms
  
  // render buffers
}

//end scene

I've never really dug into c# but looking at your source code, it looks like the transforms are packed into a constant buffer in your update function.