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