Web Forms :: Adding Sub-Properties Into Property Sheet Of A New Web-Control?

I'm having difficulties with adding sub-properties into Property sheet.

for example:

let's say I've created a web-control with a textbox in it. I can add to my web-control a new property such as:

public TextBox MyTextBox
{
get
{
return m_TextBox;
}
set
{
m_TextBox = value;
}
}

and this will be enough for me to add the TextBox into the property sheet of the web control each time i use this web-control.

My problem is with adding the TextBox properties into the property sheet with the sign "+" next to the "MyTextBox" property.

I'm using VS2008 if it's make the difference.