How do I programmatically assign a GameObject to a prefab?

When importing a mesh, I want to automatically create a prefab for that mesh, and assign that mesh to the new prefab. I have something like this:

public class MyAssetPostprocessor : AssetPostprocessor
{
    public void OnPostprocessModel(GameObject gameObject)
    {
        if (assetPath.EndsWith("_model.fbx"))
        {
            string prefabPath = assetPath.Replace("_model.fbx", ".prefab");
            Object prefab = EditorUtility.CreateEmptyPrefab(prefabPath);
            // prefab.AddGameObject(gameObject); ?
        }
    }
}

But I don't know how to add the mesh to the prefab. In the editor, I can drag-and-drop it in the project window. How do I do this programmatically?