I want to Create One to Many Relationship | with If Condition | (Category has many Subcategories)

I want to create a Category, if they have a parent categoryId then update the parent id, and categoryName on the subcategory Table. if they have not parent id then, it saves on the parent Category table with category name. Can you Correct my Relationship Code? I think it is wrong. and Very Thank you to Help me

in category Model (parent)

class Category extends Model
{
    public function subcategories(){
        return $this->hasMany(Subcategory::class);
    }
}

In the Subcategory Model,

class Subcategory extends Model

    {
        public function category(){
            return $this->belongsTo(Category::class);
        }
    }

In controller,

public function store(Request $request){
    //dd($request->categoryid);
    if($request->categoryid){ 
       $subcategory = new Subcategory();
       $subcategory->name = $request->subcatname;
       $subcategory->save(); 

     $category = $request->categoryid;
       //dd($category);
       $subcategory->category()->attach($category); 

    } 
       $category = new Category();
       $category->name = $request->catname;
       $category->save();
    }

in dd($request)

  +request: Symfony\Component\HttpFoundation\ParameterBag {#44 ▼
    #parameters: array:3 [▼
      "_token" => "z8kJy323B1Af27dKfHRHRHW1ON9NKHuP8DmYW06e"
      "subcatname" => "Samsung"
      "categoryid" => "1"
    ]
  }

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