Hello @Azure4,

Great question!

Since it sounds like you are using a third-party signup form, I would first suggest seeing if this third part signup form has built an integration with Klaviyo in order to collect these subscribers in a specific Klaviyo list. If this third-party signup provider does not have an integration with Klaviyo, I would recommend creating a redirect as described in the How to Redirect Existing Signup Forms to Klaviyo Help Center article to add these subscribers to a Klaviyo list. 

To retarget these subscribers using a flow, you can create a flow which is triggered on the list these customers are being added to, also more commonly known as a Welcome Series flow. Some Help Center articles to get you started can be found below. I can also advise taking a look at a number of Community post surrounding creating a Welcome Series flow as well!

• Getting Started with Flows
• Guide to Creating a List-Triggered Flow
• Creating an Email Welcome Series

I hope this helps!

David