How to set custom fields attribute URL unique - osclass

Hi All


My name is Dev. I am using osclass.3.8.0 and I am trying to create custom field for URL or website on publish form and want to keep it unique in the database with an error message if it is not unique while submitting the form,

I am new to php though I am almost done with setting up this theme as per requirement on my localhost but this is what I am not able to do. Any help is appreciated. Thanks