Setting properties during design

So I think this is possible but I have absoutly no idea how to do it.
I have a WebControl class called ContentFrame that has a Header property and a Content property.

  public WebControl Header {get; set;}
  public WebControl Content {get; set;}

Now let's say that I have form x and ContentFrame is on the form. What I want to do is be able to set the the Header and Content properties during design time in the source
<cc1:ContentFrame ID="ContentFrame1" runat="server">
                <Header>
                    <WebControl></WebControl>
                </Header>
         &nbs ...