Here we extend a textBox with new property

public class MyOwntextbox : TextBox
{
    [Bindable(false)]
    [Category("Properties")]
    [DefaultValue("")]
    [Localizable(true)]
    public string MyTipPreText 
    {// This property will be available in Properties window - design mode
        get
        {
            return tipPreText;
        }

        set
        {
            tipPreText = value;
        }
    }

    public string tipPreText;

    protected override void OnMouseHover(EventArgs e)
    {
        System.Windows.Forms.ToolTip t = new ToolTip();
        t.SetToolTip(this, tipPreText + " is my Tipss");
    }
}