You may try this:
Dim _ws_call_create_request As New CallCreateRequest
...
Dim _d0 As New CustomParamData
_d0.Key = "EXTFLDV_500102"
_d0.Value = "12345"
Dim _ws_cust_data_list As New List(Of CustomParamData)
_ws_cust_data_list.Add(_d0)
_ws_call_create_request.aCustomParameters = _ws_cust_data_list.ToArray()
...
Dim _ws_call_create_response As New CallResponse = _ws.CallCreate(_ws_call_create_request)
' 500102 - the custom field reference number (check in the table SU_FIELD).