Zotero provides Statute and Bill item types, but it does not tend to recognize these formats from your browser.
Instead, you can save it as a Webpage, but you will still need to manually edit most of the item information in Zotero (see below). However, Zotero will save the URL and a snapshot of the webpage, so this may still be a helpful step for you in documenting your research.
Once you have a statute saved as an item (or have manually started a new item), change the Item Type to Statute. You can then fill in the remainder of the fields.
Note: The fields Zotero provides for legislation are based on the American legal system. It may be easier to simply enter the rest of the McGill Guide citation in the Public Law Number field to ensure that the citation generates properly in your document later. Always double check to ensure that your citation is correct!
Check out Zotero's Support page for more tips and tutorials.
York University Library's Zotero guide has additional information that is not discipline-specific.