Recently started using the bulk API and had a similar issue.  For a bulk lead import that's not for the default partition, you also need to specify a static list ID to hold the results of the import (even if the file is updates only).  An URL with query string would be: /bulk/v1/leads.json?format=csv&lookupField=email&partitionName=MyPartition&listId=101   ... View more

The body literally has to be a JSON string -- just getting into using the API myself.  Please look at the docs here: https://developers.marketo.com/rest-api/bulk-extract/bulk-activity-extract/   And the request needs to be submitted as a POST, not GET.  This literally worked for me in Postman: body ---------- { "format": "CSV", "filter": { "createdAt": { "startAt": "2020-02-01T00:00:00Z", "endAt": "2020-03-01T00:00:00Z" }, "acitivyTypeIds": [ 113 ] } }   response ----------- {     "requestId": "13b9#170a17da0c4",     "result": [         {             "exportId": "ae60fc08-cd1b-46ed-8905-c84b858b1c66",             "format": "CSV",             "status": "Created",             "createdAt": "2020-03-03T17:42:09Z"         }     ],     "success": true }   Good luck! ... View more