External Id on document creation in version 16.6.3

Hi there.

I realized, while retrieving tracking information on a document an "externalid" is also delivered next to the "trackerid" in the returned json structure.

Request URL (GET):

https://<my_extream_endpoint>/v1/trackers?where_trackerid=32056E7B-2F78-1D44-AF3F-FD6E61E89AC3&guid_format=false

Response body:

{

    "status": "success",

    "data": [

        {

            "containedtypes": [

                "3A354A84-0B61-4EA2-9F7F-FF7713B0C52D",

                "CEEEDC80-FA9E-40D8-BF16-A538807BBF95"

            ],

            "expiringtime": "2021-11-26T14:40:08.130Z",

            "lastupdatetime": "2021-11-26T13:40:08.140Z",

            "trackerid": "32056E7B-2F78-1D44-AF3F-FD6E61E89AC3",

            "statusmessage": "",

            "resultval": "0",

            "externalid": "",

            "creationtime": "2021-11-26T13:40:07.270Z",

            "applicationdomainid": "salesforce",

            "sequencenumber": "3244",

            "processingstate": "5"

        }

    ]

}

My question is: How can I set such an externalid while creating a document through the Communications API so that I can filter the above query for it afterwards (i.e.: ...?where_externalid=<my_external_id>))?

I couldn't find anything related to it in the documentation. I tried to set externalid as e header as well as a property in the body's json content while posting the document generation with Postman but no success.

Thanks, Stefano