Thank you for writing.
RadMarkupEditor does not have a design-time support since it is a predefined user control. So you can make your changes in code just like you did.
Let me know if you have any other questions.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!