Filter Query by any field, not just defined available filters

Question

PS: my GraphQL skills are pretty basic so sorry for any incorrect use of words and terms

I want to achieve filtering on the code field highlighted below

(transactions --> edges --> node --> header --> transactionSource --> code = "something"

    {
  transactions(last: 10) {
    edges {
      node {
        amount
        periodId
        header {
          owner {
            owner {
              code
              description
              dbId
              ownerDbId
              ownerCode
            }
          }
          transactionSource {
            code
          }
        }
      }
    }
    pageInfo {
      hasNextPage
    }
  }
}

The client i'm working with have defined a list of filtering options, which I can successfully filter on, but when I try to filter on the code field I get the following result:

{
  "errors": [
    {
      "message": "Argument 'filter' has invalid value. In field 'code': Unknown field.",
      "locations": [
        {
          "line": 2,
          "column": 26
        }
      ],
      "extensions": {
        "code": "ARGUMENTS_OF_CORRECT_TYPE",
        "codes": [
          "ARGUMENTS_OF_CORRECT_TYPE"
        ],
        "number": "5.6.1"
      }
    }
  ]
}

I presume this is because of me not knowing exactly how to set up the filter correctly. Is there a way to filter on any field or do I need to talk to the guys maintaining the client and ask them nicely to make the code field available for filtering?

Thanks