C# - How to display a checkbox when the user has scrolled to the bottom of a rich text box

Question

So, I am trying to have a user scroll to the bottom of my rich text box and once they hit the bottom, I want the checkbox to become visible. I have searched online and tried a few solutions and they haven't worked yet. Any assistance would be helpful. Thank you

using System;
using System.Windows.Forms;
using System.IO;
using System.Text;
using System.Runtime.InteropServices;
using System.Drawing;
using Microsoft.Office.Core;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Linq;
using EUSAA;
using System.Threading.Tasks;

namespace EUSAA
{
    public partial class frmMain : Form
    {
        public frmMain()
        {
            InitializeComponent();

            txtDateTime.Text = DateTime.Now.ToString();
            txtUsername.Text = 
System.Security.Principal.WindowsIdentity.GetCurrent().Name;
            btnClose.Visible = false;
            chkAgree.Visible = false;
            rtbSecurityAwareness.LoadFile("Security Awareness.rtf");
        }

        private void chkAgree_CheckedChanged(object sender, EventArgs e)
        {
            if (chkAgree.Checked)
            {
                btnClose.Visible = true;
            }
            else btnClose.Visible = false;
        }

        private void btnClose_Click(object sender, EventArgs e)
        {
            using (StreamWriter sw = File.AppendText(@"I:\Security Awareness 
Signed Users\Signed Users.txt"))
            {

sw.WriteLine(System.Security.Principal.WindowsIdentity.GetCurrent().Name + 
"\t" + DateTime.Now.ToString());
            }
            Environment.Exit(0);
        }

    }
}