CueTextBox.cs

using System;
using System.ComponentModel;
using System.Windows.Forms;
using System.Runtime.InteropServices;

class CueTextBox : TextBox
{
    private string _cue;

    [Localizable(true)]
    public string Cue
    {
        get => _cue;
        set
        {
            _cue = value;
            UpdateCue(); 
        }
    }

    [DllImport("user32.dll", CharSet = CharSet.Unicode)]
    private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wp, string lp);

    private void UpdateCue()
    {
        if (IsHandleCreated && _cue != null)
        {
            SendMessage(Handle, 0x1501, (IntPtr)1, _cue);
        }
    }
    protected override void OnHandleCreated(EventArgs e)
    {
        base.OnHandleCreated(e);
        UpdateCue();
    }
}