Sorry, I can't reply in French, but I think I can answer your question: It's not possible to insert any additional HTML or script language content into a website based on Magix Website Maker, which I understand you're enquiring about. Website Maker is designed in a way that all editing takes place exclusively through the graphical, browser-based user interface.
If you're participating in some kind of affiliate program and you want to place some kind of link and advertisement, your best option is probably to place the textual content of image on your site by uploading it into Website Maker and attaching a link to it via the "Linking" function in the "Properties" panel. This might do the trick, since the affiliate code you were given is probably just a combination of displaying some image/text and hotlinking to your partner site.