We would recommend taking a look at the the Weebly App center for a way to add email subscription pop ups to your site. You can check the Weebly Apps here:
Note: These apps are created by third-party developers and act as extensions to the Weebly platform. For this reason, we do not offer support for these third-party applications. If you need assistance with these apps, you will want to reach out to the app developers directly. Some apps also have an additional cost associated with them. This cost is not included in your Flipcause subscription and will be paid directly to the developers.
If you have MailChimp, you can add a MailChimp pop up to your website: https://kb.mailchimp.com/lists/signup-forms/add-a-pop-up-signup-form-to-your-website