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You need something that places a cookie on the visitor's computer. Web Designer cannot do this.

However if you can find a script that adds a cookie to your visitor's computer then you can add the script to Web Designer as a "placeholder object".

Here is a link to a Google Search for Cookie Scripts. If you find something here that works, and that generates a script, then customize the script as you wish, copy the script, and select a placeholder object (a simple rectangle that you create with the Rectangle Tool), open the Website Properties > Placeholder section and paste the script into the Replace with HTML Code window.