Microsoft Graph Webhook Missing ClientState

I am having trouble starting a Microsoft Graph webhook subscription (for a mailbox in particular). When initiating the subscription it appears as though Microsoft accepts all of the parameters I am sending to configure the subscription but fails the total subscription because it is receiving a non 2xx response from the endpoint I have configured.

The reason my endpoint is sending a 401 back to Microsoft is because their POST that includes the subscription validation token is missing the clientState.

I am using the clientState key-value pair to authenticate all the communication between Microsoft and my endpoint. If my endpoint does not see the correct clientState it will return a 401.

Any ideas on what I might be missing or if I should go about this in a different way? In my opinion allowing my endpoint to accept unauthenticated GET/POST's is not an option.

Example request body using POST method including the API key in the header:

{
    "changeType": "created",
    "clientState": "testClientState",
    "resource": "users/<UserName>/messages",
    "expirationDateTime": "2017-08-10T10:24:57.0000000Z",
    "notificationUrl": "<EndpointURL>"
}

Error Returned from Microsoft:

"error": {
    "code": "InvalidRequest",
    "message": "Subscription validation request failed. Must respond with 200 OK to this request.",
    "innerError": {
        "request-id": "adf7fc7b-6b14-4422-8526-c1391be8dd27",
        "date": "2017-08-07T16:24:59"
    }
}

I understand everything to work as intended until my endpoint is sent the validation token because I receive the validation token but my endpoint rejects it because it is missing the client state.

Endpoint Log Snippet:

queryStringParameters": {
    "validationToken": "<ValidationToken sent by Microsoft>"
}

I am basing my API endpoint logic off of some of Microsoft's developer guides. For the subscription creation in particular I am using this guide.

It appears this question was also asked but not answered on GitHub.