curl --request GET \
  --url https://api.withflex.com/v1/coupons/{id} \
  --header 'Authorization: Bearer <token>'

{
  "coupon": {
    "coupon_id": "fcoup_1234567890",
    "amount_off": 123,
    "applies_to": {
      "products": [
        "<string>"
      ]
    },
    "duration": "<any>",
    "duration_in_months": 123,
    "metadata": {},
    "name": "<string>",
    "percent_off": 123,
    "max_redemptions": 123,
    "times_redeemed": 123,
    "valid": true,
    "created_at": "<string>",
    "test_mode": true
  }
}