Request a definition of all user defined free fields for registering Relations.

parameters:

example: https://backend.ikgagolfen.nl/igg/relationfreefielddefinition?auth=<your authrisation code here>&crsnr=1

result:

{
  "relationfreefielddefinition": [
    {
      "ffdnr": 1,
      "ffdname": "contractdatum",
      "ffdformat": "LocalDate"
    },
    {
      "ffdnr": 8,
      "ffdname": "soort contract",
      "ffdformat": "listbox",
      "optiontable": [
        {
          "ffonr": 47,
          "ffoname": "basic contract"
        },
        {
          "ffonr": 48,
          "ffoname": "wurg contract"
        }
      ]
    },
    {
      "ffdnr": 2,
      "ffdname": "datum einde contract",
      "ffdformat": "LocalDate"
    },
    {
      "ffdnr": 3,
      "ffdname": "contractdocument",
      "ffdformat": "binary"
    }
  ]
}

If you look at the the 2nd record in the resultset, you see that the value is to be selected from a list of predefined options.