@bhas 

Form exclusions can only be based on Klaviyo personal profile data, URLs on YOUR website, or Country location.

If these are new people who don’t already have a profile within your account, Klaviyo won’t be able to identify them to exclude them based on the URL they came from. 

Are you able to send this traffic to a specific landing page dedicated to them? Then you can exclude your regular pop-up from that page.

Also, are you having them sign up for your list to get the discount? If you do that, once they have a Klaviyo profile, as long as you have existing profiles excluded from your pop-up they won’t see the regular one when they navigate to the rest of your site.

You could also exclude them based on their last referring domain if you wanted to still show your main pop-up to existing profiles for some reason. But you can’t collect that last referring domain unless they have a profile in your Klaviyo account.