Question about Graph API - changes notification

Hello, I'm writing a java project to create subscription to online meeting resources. I followed this instruction https://docs.microsoft.com/en-us/graph/webhooks?view=graph-rest-1.0

to construct a new subscription. This is my request:

POST https://graph.microsoft.com/beta/subscriptions
Content-Type: application/JSON
change Type": "created, updated"
"notificationUrl": "https://my.ngrok.url/notificationClient",
"resource": "/communications/onlineMeetings/{meetingID}",
"expirationDateTime": "2016-03-20T11:00:00.0000000Z",
"clientState": "SecretClientState"

And I created a project locally to receive the POST request as follow

@PostMapping(path = "/notificationClient")
public ResponseEntity<String> Notification(@PathParam ("validationToken") String opaqueTokenCreatedByMicrosoftGraph)

I should receive request from Microsoft Graph like this:

Content-Type: text/plain; charset=utf-8 POST https://{notificationUrl}?validationToken={opaqueTokenCreatedByMicrosoftGraph}

However, the validationToken I received is null. Is there anything wrong and how to create a subscription to online meeting resources?