How I can attach Object to Prefab

You can't rearrange children of a prefab directly. Don't think you can even rename them. All you can do is replace the whole thing, but there is a trick:

To make major changes to a prefab: drag the prefab into your scene, to get a copy you can play with. Make any changes there. It might say "losing prefab connection" when you change children -- just click OK. Then drag that copy back to the prefab. The prefab should immediately "update" with those changes. (Then delete the copy.)

If you gave it a different main parent, then dragging back will warn you "possibly unwanted prefab replacement." Usually that means you messed up, and dragged into the wrong place, but in this case its fine -- just double-checking you really want a new parent.

Hey bud, im not entirely sure as to what you are trying to achieve here exactly, but....

The image in capture 3 denotes you have created a prefab in the project window and then dragged in your Cube_A to it.

Instead, delete everything in your scene and then create your cube object, after you have created your cube object, drag it, from the hierarchy window down in to your Project/ Assets window, you will see it turn blue in the hierarchy panel and it is now considered a prefab in your project. if you want to create a prefab object that holds your cube object, then what you have already done is correct.... If you just want to prefab the actual Cube object , simply drag that into the project panel, not into a prefab you created first as it will become a child of that prefab rather than a self contained entity of its own(having no parented object to sit inside)