Well, it needs one of those products in the cart. Also, you need to remove those shipping method restrictions. Instead configure the free shipping method so it requires a coupon.
I thought the pro version allowed you to put a product in the basket later, and have the coupon apply later…
So I remove the shipping methods from the checkout tab to make it work?
Do you mean an automatic coupon? Yes, that’s possible. In that case you don’t need to provide ?apply_coupon in the url. Or, if you want the coupon only to be applied if someone uses that url or applies it manually, untick ‘auto coupon’ and tick ‘allow apply when invalid’.