The onclick event of all links is disabled on purpose in Design mode. If the links are clickable this will erase the content entered by the user because the link will load a web page in the content area. After that the user will be unable to restore the entered content.
What you can do is to open the clicked link in a new window. The following articles are helpful for this scenario: Opening Link In A New Window When Clicked
and Opening Link In A New Window On Double Click
the Telerik team