Create an object and assign an existing model for it via script

Let`s say I have a 3d model in the scene called:“SpartanKing” and I want to add an object for it in script and also to assign the SpartanKing model for it via script because I do not want to assign it in the Unity scene by dragging and drop it in the public GameObject variable

I want to acces the GameObject from script

Why not use prefabs? Make once in editor, replicate when-ever in-game.

Your use of ‘object’ here is confusing to me.

To Create a new object in the scene using an existing object or a prefab as Doireth mentions you use the Instantiate() method.

As for accessing one game object from another, Here is a reference page on accessing other game objects.