How we write a script to trigger after user clicking extension field (data type url)

Hello all ,

    Now we have created extension field (data type :url) as below picture

Problem -> In display mode , the status of this link is set to "read only" but user can click the link and redirect to another page.Actually user can't action anything in display mode.

Question -> How we write a script to trigger after user clicking extension field (data type :url)