Hi @1982luca ,
You should never put a pixel onto a 3rd party site without the consent of the owning webmaster. If you are working with the webmaster, it is best to ask them to put the pixel on themselves. If they have otherwise given you access to edit their site (which I wouldn't consider 3rd party at that point), you could put the pixel on in different ways. The most flexible would be using a tag manager, e.g. Adobe Launch. This way you can update the pixel code or add/remove additional pixels while only requiring the website(s) to maintain a single script tag. Furthermore, if you do use Launch, you need only use a rule with a core, custom code action to place the javascript/html needed to fire an additional pixel.