tam istediğin gibi sanırım başka bir thread daha koyup başka bir işte yaptırabilirsin.
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 WindowsFormsApp1
{
public partial class Form1 : Form
{
public Form1()
{
Control.CheckForIllegalCrossThreadCalls = false;
InitializeComponent();
}

private Random r;
private void Form1_Load(object sender, EventArgs e)
{
r=new Random();

}


private void button1_Click(object sender, EventArgs e)
{
Thread thread1 = new Thread(t =>
{
for (int i = 0; i < 100; i++)
{
int a = r.Next(0,10000);
label2.Text = a.ToString();
Thread.Sleep(1000);
}
})
{ IsBackground = true };
thread1.Start();
}

}


}