C# Quartz MessageBox Block UI

Question

1. How can I block main UI Thread by MessageBox.Show("test") ?
2. How can I change text my label from executed code by Quartz?

txtLabel.Text = "new value"; <= this not working because txtLabel don't exists on this class.

using System;
using System.IO;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
using Quartz;

namespace EMoreauDemoQuartzCS
{
    [DisallowConcurrentExecution]
    public class JobWriteToTextFile : IJob
    {
        public static Thread t;
        public void run()
        {
            MessageBox.Show("test");
            txtLabel.Text = "new value";
        }
        public Task Execute(IJobExecutionContext context)
        {

            t = new Thread(run);
            t.SetApartmentState(ApartmentState.STA);
            t.Start();
            t.Join();
            return Task.CompletedTask;
        }

    }
}