Like this:

 public static DependencyProperty __ListItemProperty = DependencyProperty.Register("__ListItem", typeof(int), typeof(AddSPWeb));

        [Description("Id of the list")]
        [Category("Custom")]
        [Browsable(true)]
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
        [ValidationOption(ValidationOption.Required)]
        public string __ListId
        {
            get
            {
                return ((string)(base.GetValue(__ListIdProperty)));
            }
            set
            {
                base.SetValue(__ListIdProperty, value);
            }
        }