For a successful top-up, you need to specify in the request body

Parameter	Required	Description
amount	yes	Deposit amount in USD,ranging from 0.1 to 1000
username	yes	User login steam
reference	no	your ID, if necessary

If you follow the above rules, you will receive a response like this:

{
  "transaction_id":"uuid",
  "status": "pending",
  "error_code": 0
}

This means that your Steam account top-up is being processed. You can read more about statuses Statuses

If you receive a message such as:

{
  "error_code": -1
}

Review the list of errors Error Codes