31C3 | Personal Tracking Devices Preso | Runtime Evironment library

I am trying to subscribe to a user's mailbox using Microsoft Graph subscriptions. I try to subscribe to a specific user's mailbox using the 'created' changeType only, which is being accessed via a token created using client credentials on my company's tenant in AD (app registration). This is being done in my dev environment (Laravel framework, using Laravel Valet + ngrok).

I can successfully create the subscription on MS's side, making the notification URL the ngrok URL and I am able to pass validation etc, but MS does not appear to return confirmation that the subscription has been created with a "201 Created" response like it says it should in the docs.

I can confirm myself that the subscription works because I can send an email to that mailbox and ngrok catches the request containing the correct information - just the initial setup of the webhook does not return 201 on successful creation.

I would like to have this confirmation so I can store the ID of the webhook etc in the application for further use (updating expiry on cron etc).

Apologies if I haven't submitted enough information to make the question clear - just ask for relevant details.

How should I proceed with this?

EDIT - Including code examples and routing

The code I use is a custom class so I cannot include everything but this is how it gets built:

Microsoft.php

I've iterated of lots of articles - for some reason I couldn't find any which describes what must be a simple procedure: How to combine a calculated CRC with the original message, so that calculating CRC again results in 0 (=checks out as correct)? I did find several examples with 'longhand' calculations (only 2 or 3 bit CRCs), but no example which uses a library function such as [crcmod][1] (Python library).

Here's a simple program I wrote to check it out: