From the website:
Receive $100/$150/$200 Costco Shop Card when you purchase a set of 4 or more selected Michelin Tyres when fitted at Tyre Center.
I believe you can only order on the website or in-store and not on the app. If you do it on the website, you get the gift card emailed to you in a week (emails are processed every Monday, so look out for emails on Tuesdays). If you purchase in-store, you get the physical gift card straight away.
I ordered tyres today from a tire shop and got the gift card about two hours after my order.