curl --request POST \
  --url https://api-v2.missioninbox.com/billing/validate_coupon/ \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: application/json' \
  --data '{
  "coupon_code": "<string>"
}'

{
  "valid": true,
  "coupon_id": "<string>",
  "percent_off": 123,
  "amount_off": 123,
  "error": "<string>"
}