How to define default “TagPrefix” for custom controls

[ToolboxData("<{0}:MyDropDownList runat="server" CustomProp="123"></{0}:MyDropDownList>")]
public class MyDropDownList: DropDownList
{
   ... code here
}

<%@ Register Assembly="DBMClientPortal.Controls" Namespace="DBMClientPortal.Controls"
    TagPrefix="cc1" %>

[assembly:TagPrefix("MyControls","RequiredTextBox")]