Unfortunately, Klaviyo doesn't directly tag customers who subscribe via the Shopify checkout (not forms) due to a recent update in December 2022. Previously, Klaviyo relied on Shopify's "Accepts Marketing" tag, but now it focuses on Shopify's subscription model.

Here's a workaround using Shopify customer tags:

1. Create a Shopify Tag: In your Shopify admin, create a tag named "Checkout Subscriber."
2. Shopify Checkout Settings: Explore options in your checkout settings to automatically tag customers who subscribe to your email list during checkout (functionality might vary depending on your theme or apps).
3. Klaviyo Segmentation: Create a segment in Klaviyo targeting customers with the "Checkout Subscriber" tag.
4. Klaviyo Flow Trigger: Trigger your Klaviyo flow based on this segment to target only checkout subscribers.

This approach leverages Shopify's existing functionalities and requires minimal technical setup. Test thoroughly to ensure it works as expected.