Prepaid RTR products always REQUIRE a user input like a phone number or email, in this case, for example, the product have an input labeled “Phone Number”. You need build a form in the UI to allow user input required information. See more about user inputs

Firstly must get all products inputs required to make the refill.

query ($id: ID!) {
  node(id: $id) {
    ... on Product {
      isPIN
      inputs {
        name
        type
        required
        label
        html {
          type
          maxlength
          minlength
          min
          max
          pattern
        }
      }
    }
  }
}
{
  "id": "4132Q0YzBh4PQ=="
}
{
  "data": {
    "node": {
      "isPIN": false,
      "inputs": [
        {
          "name": "accountNumber",
          "type": "phone",
          "required": true,
          "label": "Phone Number",
          "html": {
            "type": "tel",
            "maxlength": null,
            "minlength": null,
            "min": null,
            "max": null,
            "pattern": null
          }
        }
      ]
    }
  }
}

Then submit the transaction with all user inputs

mutation ($input: SubmitTransactionInput!) {
  transactions {
    submit(input: $input) {
      transaction {
        date
        status
        grossTotal
        adjustments {
          name
          total
        }
        total
        response {
          ... on PinSaleResponse {
            pin
            code            
          }
          message
          redemptionUrl
        }
      }
    }
  }
}
{
  "input": {
    "product": "4132Q0YzBh4PQ==",
    "inputs": { "accountNumber" : "+12345678901" }
  }
}
{
  "data": {
    "transactions": {
      "submit": {
        "transaction": {
          "date": "2020-12-16T13:09:43-05:00",
          "status": "COMPLETED",
          "grossTotal": 3.89,
          "adjustments": [],
          "total": 3.89,
          "response": {
            "redemptionUrl": null,
            "message": "Your account has been successfully credited."
          }
        }
      }
    }
  }
}

The response has more properties than shown here, this is a basic example, to see all possible properties look for the SubmitTransactionPayload type object in the explorer