@model WebApplication1.Controllers.PersonViewModel
@{
    ViewBag.Title = "Index";
}

<h1>@ViewBag.Title</h1>

@using(Html.BeginForm())
{
    @Html.LabelFor(x => x.FirstName)
    @Html.TextBoxFor(x => x.FirstName, new { @class = "form-control" })

    @Html.LabelFor(x => x.LastName)
    @Html.PasswordFor(x => x.LastName, new { @class = "form-control" })

    @Html.LabelFor(x => x.Address.City)
    @Html.TextBoxFor(x => x.Address.City, new { @class = "form-control" })

    @Html.LabelFor(x => x.Address.Number)
    @Html.TextBoxFor(x => x.Address.Number, new { @class = "form-control" })

    @Html.LabelFor(x => x.Address.StateId)
    @Html.DropDownListFor(x => x.Address.StateId,
        Model.States?.Select(x => new SelectListItem
        { Text = x.Name, Value = x.Id.ToString() }), "--Wybierz--",
        new { @class = "form-control" })

    <button class="btn btn-primary">Klik</button>
}


using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace WebApplication1.Controllers
{
    public class HomeController : Controller
    {

        public ActionResult Index()
        {
            var model = new PersonViewModel
            {
                FirstName = "Jan",
            };

            InternalIndex(model);

            return View(model);
        }

        [HttpPost]
        public ActionResult Index(PersonViewModel p)
        {
            InternalIndex(p);

            return View(p);
        }

        private PersonViewModel InternalIndex(PersonViewModel p)
        {
            p.States = new[]
            {
                new StateItem(1, "śląskie"),
                new StateItem(2, "mazowieckie"),
                new StateItem(3, "małopolskie"),
                new StateItem(4, "pomorskie"),
                new StateItem(5, "warmińsko-mazurskie")
            };

            return p;
        }
    }

    public class PersonViewModel
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public AddressViewModel Address { get; set; }

        public IEnumerable<StateItem> States { get; set; }
    }

    public class StateItem
    {
        public int Id { get; set; }
        public string Name { get; set; }

        public StateItem()
        {
        }

        public StateItem(int id, string name)
        {
            Id = id;
            Name = name;
        }
    }

    public class AddressViewModel
    {
        public string City { get; set; }
        public int Number { get; set; }
        public int StateId { get; set; }
    }
}