Opt-in creation platforms: All opt-in platforms will include the success metrics of your pop-ups. You will know how many website visitors saw, ignored, or engaged with your lead capture strategies. These are great metrics to see side-by-side when testing multiple versions of opt-ins.  Google Analytics: You can create google analytics goals surrounding pop-up actions. This is a great way to assign conversion value to each goal achieved through pop-ups. It also allows you to see the results of all of your marketing efforts in one house, instead of needing to constantly hop from Google Analytics to the third-party platform.  Lead Generation: Make your opt-in incentives gated. That means every individual that wants the incentive your pop-ups offer need to give you their email. Not only is this a great way to track the interest in your offer, but you can use these email leads for future campaigns.