Adding a custom link on an out of the box Portlet

Document ID : KB000113649
Last Modified Date : 12/09/2018
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We have a business requirement where we need to add a custom attribute to an out of the box portlet which will have link (url) to our custom object instance and should contain same value for every instance. How can we accomplish that?
Making a URL attribute read only with a default value to the custom object properties worked.