I need to create a conversion tag, to detect the click on the "subscribe newsletter" button.
jQuery Selector: button.newsletter-subscription__form__button.theme-bg-color-primary
Please dont try to add the script using extension. Try using Tealium generic tag for custom implementation
if the required tag is not available in the tag market place.
The tag which you are looking for is available in the tag marketplace named : MediaMath (Adroit Digital).
Then in the extension which you have added, modify the tracking event to 'link' and map to the created UDO variable 'event_name'. Just for an example I have given the condition as 'subscribenewsletter', you can change according to your requirement
In the load rule you can define the condition that tag should fire when the event named subscribenewsletter occurs