In the article, we use Select Tag Helper in ASP.NET Core 2.1, I went online and saw many people also asking about how to use this, now I share with everyone how to use it.
Example : We already have a Categories table that alreadly has data available, and have create a Category.cs class in Models folder, you can the article previous:
Create Database using Code First in ASP.NET CORE 2.1
The first, you need add using Microsoft.AspNetCore.Mvc.Rendering in Controller file , Okay, go to Cotrollers folder, click right->add new->Controllers-> create ProductController.cs , after then add (Microsoft.AspNetCore.Mvc.Rendering)
Create Action CategoriesList() in ProductController.cs file

public ActionResult Create()
        {
            CategoriesList();
            CategoriesList2();
            return View();
        }

 private void CategoriesList(object selectCategory=null)
        {
            ViewBag.categories = new SelectList(_db.Categories.ToList(), "idCategory", "Name",selectCategory);
        }

private void CategoriesList2()
        {
            var data = _db.Categories.ToList();
            List<Category> list = new List<Category>();
            foreach(var item in data)
            {
                list.Add(new Category
                {
                    idCategory = item.idCategory,
                    Name = item.Name
                });
            }
            ViewBag.Categories2 = list;
        }

_db.Categories.ToList(): read data from Categories table in Database
SelectList(): if you use SelectList(), you can insert library (using Microsoft.AspNetCore.Mvc.Rendering) in ProductController.cs file Okay, we need to Views/Product/ folder, create Create.cshtml file

<div class="form-group">
                    <label>Select Category</label>
                    <select asp-for="idCategory" asp-items="@ViewBag.Categories" class="form-control">
                        <option>Select one</option>
                    </select>
                </div>
                <div class="form-group">
                    <label>Select Category</label>
                    <select asp-for="idCategory" asp-items="@(new SelectList(ViewBag.Categories2,"idCategory","Name"))" class="form-control">
                        <option>Select one</option>
                    </select>
                </div>

If you want to use it for Edit.cshtml, it is similar to Create.cshtml:
Add Action Edit(int id) to ProductController.cs file

public ActionResult Edit(int id)
{
      CategoriesList();
      CategoriesList2();
      return View(_db.Products.Find(id));
}

The Article : Select Tag Helper in ASP.NET Core MVC