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

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private Thread _t;
        private void button1_Click(object sender, EventArgs e)
        {
            _t = new Thread(MyProc);
            _t.Start();
        }

        private void MyProc(object obj)
        {
            var myForm = new Form2();

            myForm.SetText("Привет из второго потока");

            myForm.ShowDialog();
        }
    }
}

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

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private Thread _t;
        private void button1_Click(object sender, EventArgs e)
        {
            _t = new Thread(MyProc);
            _t.Start();
        }

        private void MyProc(object obj)
        {
            while (true)
            {
                var MSG = DateTime.Now.Ticks.ToString();

                if (InvokeRequired)
                    Invoke(new UpdateUiDelegate(UpdateUi), new object[] { MSG });
                else
                    UpdateUi(MSG);
            }
        }

        private delegate void UpdateUiDelegate(string msg);
        private void UpdateUi(string msg)
        {
            label1.Text = msg;
        }
    }
}