Free Gift With Purchase Using Code (to track purchases) No discount

Hi, we want to run a social media add for a free gift with purchase. The consumer would receive 2 pre-selected items for free on orders over $250. We want to be able to track the success of the social media ad, so we don’t want this to be a blanket offer to just anyone who orders on the website.

Through Meta we can see for example, that the ad reached (made up numbers) 10,000 people and there were 500 clicks to the website…but we want to see that 25 people actually made a purchase and I feel like using a code would show that.

How can I set up a coupon code that doesn’t apply a discount, but just so that the order is notated with the code, indicating that they are ordering because of the social media ad??