Hi @Yves,
The Universal Website Tag is meant for use across the entire website where tailored audiences or conversion events can be tracked using URL rules such as URL contains: “/products/glassware” to accumulate audience or know that users are hitting that part of your site after engaging with ads.
In the case where you want to track a button click. The single event tag is the right method to track that action. Single event tags don’t need a URL in order to track, they track when the code executes. I assume the button click can execute a function onclick containing either JS or image pixels, which would solve your ask.