Please add following code in newsletter-form.liquid file as shown in the screenshot
<input type="hidden" name="contact[tags]" value="newsletter">
Source: https://help.klaviyo.com/hc/en-us/articles/115005080407-Integrate-with-Shopify
Please add following code in newsletter-form.liquid file as shown in the screenshot
<input type="hidden" name="contact[tags]" value="newsletter">
Source: https://help.klaviyo.com/hc/en-us/articles/115005080407-Integrate-with-Shopify