If you cannot find the email from Shaper Tools that included your promotional offer for Fusion 360, you can follow the link found below. You may also want to check your spam filters and make sure that you have whitelisted all emails originating from the domain @shapertools to ensure that you receive future communication from Shaper Tools.
Note that the offer is redeemable only one time for each email address registered with Autodesk.
If you use Fusion 360 as a hobbyist, or if you use it for a business with an annual revenue that is less than $100,000, you can use Fusion 360 for free under their very generous hobbyist licensing agreement. You can find more information here