Indeed, the Target property takes effect if the ButtonType is set to LinkButton,
this is the default RadButton behavior. You may utilize this type and set the button as image button. This is shown in the example below:
If you insist on keeping the ButtonType to StandardButton
of RadButton. This implementation is discussed in the following forum thread: http://www.telerik.com/community/forums/aspnet-ajax/button/navigateurl.aspx
the Telerik team
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