Hi @guptaanjali538, 

It sounds like you’re looking to create a single click button for customers that are logged into Shopify, which unfortunately is not supported through Klaviyo. When a customer clicks a button to subscribe without explicitly consenting through Accepts Marketing, it doesn’t qualify as consent. 
@david.to shares a really great response with another customer in the post below with more details on how you can Sync Your Shopify Newsletter Subscribers, but for convenience I’ll paste it below: 
 

By default, users who create an account on your site would not be added to a list as signing up for an account because it does not equate to consent to join a marketing list. 

To collect consent from these contacts and to allow them to join a list, you’ll want to adjust the code for this account creation form to include an area that passes Accepts Marketing with phrasing such as “Keep me up to date on news and exclusive offers”; similar to the checkbox found on the checkout page.

On top of having this phrasing and code present, you’ll also need to ensure this form is passing the hidden input group tag of <input type="hidden" name="contactutags]" value="newsletter"> as detailed in the How to Link a Shopify Signup Form to Klaviyo Community post highlighted prior. A refresher on how to apply this code can be found in the Sync Your Shopify Newsletter Subscribers to a Klaviyo List subsection of the How to Integrate with Shopify article.

Should you need assistance either custom coding this Accepts Marketing field or applying the hidden input group tag to be incorporated into your account creation form, I would recommend reaching out to a developer you are familiar with or contacting one of our many Klaviyo Partners through an extensive network of agency partners.

Thanks so much and have a great weekend! 
-Cassy