Can you please share with us the control template, or let us know if there's any specific reason why you won't surround the custom field in <a href> tag so the rendered output of your custom field will then be displayed as hyperlink? Generally one option would be to use Eval in your markup to get the value of your field, the other to declare an asp:HyperLink control int eh markup and set the NavigateUrl property in the code-behind where you can operate with the custom field. Please do not hesitate to get back to us if you need some additional information or have further questions.
All the best,
the Telerik team
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