Hey @Le Rallye du Whisky18! I'd recommend adding it as a widget somewhere that shows up on that page, like in your sidebar or footer section. Since it's a popup, it doesn't matter where the code goes precisely, so long as it loads on the page somewhere. (And if you have our WordPress plugin23, you should have access to a ConvertKit form widget as well, which helps make it easier!)
I hope that helps! Let us know if you have trouble.