Use iteration object in data copy mapping

I have dataflow which makes REST call to get different courses from API. For each course I need to get more detailed content. I have dataflow that uses Data Copy action to get all the courses and then I have For Each action that iterates every item received from API. I am able to do an another REST API call to get detailed course information, but the API response does not have the course identifier. I have it in the iteration item, but I cannot use it in mapping section. If I try to put item().id or {item().id} it produces an error:

ErrorCode=UserErrorSchemaMappingCannotInferSinkColumnType,'Type=Microsoft.DataTransfer.Common.Shared.HybridDeliveryException,Message=Data type of column 'id' can't be inferred from 1st row of data, please specify its data type in mappings of copy activity or structure of DataSet.,Source=Microsoft.DataTransfer.Common,'

Is there a way to use iteration object in mapping / should I do my dataflow in some another way? I have used the item().id in relative path, so I know it works there.