From your Shopify admin, go to Online Store > Themes.
Find the theme that you want to edit and click Customize.
In the left-hand toolbar, click Theme settings.
Platform: Specify the platform to run the newsletter feature. There are values: Shopify, Klaviyo, Mailchimp.
Show a checkbox with the link of your terms and conditions page. Customers need to tick to agree with your terms & conditions before subscribing to their email. This feature only works with the platform is Shopify.
Enable: turn on/off this feature.
Terms and conditions page: specify a static page to display your terms and conditions.
Error message: the message that is shown when customers subscribe to their email without ticking the checkbox.
Only work when you choose the platform is "Klaviyo". Add your Klaviyo API key and embed code. You can refer to https://help.klaviyo.com/hc/en-us/articles/360002035871-Install-Klaviyo-Signup-Forms to get the API key and embed code.
Only work when you choose the platform is "MailChimp". Add your MailChimp embed code. You can refer to https://mailchimp.com/help/add-a-signup-form-to-your-website to get the embed code.
Enable: turn on/off the popup.
Short text: add your text to the popup.
Background color: pick a color for the popup's background.
Image: pick or upload your image to display it in the popup. The image will not be shown on the mobile screen to help customers focus on the text and the newsletter form.