Hi @kurban93 ,
Thanks for posting your question to the Community! In order to add custom HTML to a pop-up form, it would have to not be a Klaviyo prebuilt form. In order to edit the HTML of a form you would need to use a Klaviyo Legacy sign up form. These forms will not use our new signup from builder will output the full form to HTML which can be edited.
With our legacy forms, we provide you with the form code so that you can do what you please. Take a look at our guide on our legacy form here: Add and Customize a Legacy Popup.
Hope this helped!
Alex