対象のプロパティに以下の属性を記載すればOK
[Editor("System.ComponentModel.Design.MultilineStringEditor, System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(System.Drawing.Design.UITypeEditor))]
サンプル
[Editor("System.ComponentModel.Design.MultilineStringEditor, System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(System.Drawing.Design.UITypeEditor))] public string TextBoxText { get { return this.TextBox1.Text; } set { this.TextBox1.Text = value; } }