The magnet URI scheme is designed to be interpreted by the client (your browser), and passes data to other software - it doesn't actually cause the browser to make an external connection to anywhere. Web-forwarding proxies (assuming that's the kind of proxy you're talking about) are unlikely to support protocols besides HTTP(S) for security reasons, and as already mentioned supporting magnet links wouldn't make any sense anyway.
If you're trying to mask your IP from a torrent search site (such as The Pirate Bay), then use the web-forwarding proxy to load the page containing the magnet link (or use the web-forwarding proxy to obtain the .torrent file). This is unnecessary though, if your concern is copyright law, since browsing a search site or downloading a .torrent file is not a breach of copyright.
If you're trying to mask your IP from the torrent P2P process itself, your torrent client's traffic is your concern. Based on the magnet link's data, it will try to request a copy of the torrent metadata (the stuff in the .torrent file) from one or more servers, then contact any available tracker servers, before finally connecting to other users to download the actual data. I believe some torrent clients allow for configuring a proxy server to use for these communications, or your OS might provide software for configuring a proxy gateway. In short, the way in which you obtain the .torrent file or magnet link will not prevent your IP from being used for hundreds of connections by your torrent client, unless its traffic is also channelled through a proxy (and I should that add you won't find a legitimate proxy to use for this sort of purpose for free).