Linking File Request and Incoming Message

We are using the LiquidFiles API to send File Requests. Our plan is to then import the requested files into another system. The problem we are facing is reliably knowing which incoming message matches which File Request. We could attempt to match them based on email and subject but it is not reliable especially if multiple File Requests have been sent.

Is it possible to add a relationship or property on to the message to provide an explicit reference?

The below example uses the url returned by the File Request API as the `source` property:

Code:

{
    "sender": "person@example.org",
    "expires_at": "2020-04-13",
    "expires_after": null,
    "authorization": 3,
    "size": 8756,
    "subject": "[File Request] Customised subject",
    "message": "This is the response",
    "private_message": false,
    "id": "9VO41ujd2TGRMRzLs5QuM6",
    "authorization_description": "Only Specified Recipients can access",
    "recipients": [
        "info@company.org"
    ],
    "ccs": [],
    "url": "https://files.company.com/message/9VO41ujd2TGRMRzLs5QuM6",
    "created_at": "2020-03-30T15:47:22.000+10:00",
    "source": "https://files.company.com/requests/5Y1pTibku33JvssTN6rdDE",
    "attachments": [
        ...
    ]
}