Activate a flow with those who did not complete a signup form

Thank you,
Are these first time signups? Or current list members? Sending emails to people who have not signed up for emails is not a best practice.

You could use other marketing channels like remarketing ads on Meta (Facebook / IG) or Google to target people who have visited this page with a signup message. You can even do a lead generation ad unit on Meta which integrates directly to Klaviyo. This means when people sign up for the newsletter on Meta, their data is sent directly to Klaviyo automatically. 

Here is a help article on that integration:
https://help.klaviyo.com/hc/en-us/articles/115005082127-How-to-Integrate-with-Facebook-Advertising