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To create a new snippet go to the "JS/CSS Snippets" tab tab
Click the "+ New snippet" button
Then you'll see a JS a JS or CSS redactor
Then you'll see a JS a JS or CSS redactor
See an example of the JS code and CSS styles description for a 240x400 creative tag.
If you want to check a snippet's proper functioning, first set up a dynamic creative: Adding XML feed and dynamic creative Product Feed and Dynamic Creative settings