Form1.cs

using System;
using System.Windows.Forms;

namespace WindowsFormsApp2
{
    public partial class Form1 : Form
    {
        MyClass mc = new MyClass();

        public Form1()
        {
            InitializeComponent();
            mc.myLabel = label1;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            mc.Value = textBox1.Text;
        }
    }
}

MyClass.cs

using System.Windows.Forms;

namespace WindowsFormsApp2
{
    class MyClass
    {
        public Label myLabel;
        public string _value;

        public string Value
        {
            get
            {
                return _value;
            }

            set
            {
                _value = value;
                myLabel.Text = _value;
            }
        }
    }
}