PraktikaGlava4/Form4.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Praktika409
{
    public partial class Form4 : Form
    {
        public Form4()
        {
            InitializeComponent();
            List<string> states = new List<string>
        {
            "Аргентина", "Бразилия", "Венесуэла", "Колумбия", "Чили"
        };

            // добавляем список элементов
            domainUpDown1.Items.AddRange(states);
        }

        private void Form4_Load(object sender, EventArgs e)
        {

        }

        private void domainUpDown1_SelectedItemChanged(object sender, EventArgs e)
        {
            MessageBox.Show(domainUpDown1.Text);
        }
    }
}