I’m trying to subscribe to my calendar events using Microsoft Graph API in Postman,
but I keep encountering a 400 error.
I believe I have the correct API permissions (Calendars.Read, Calendars.ReadBasic.All, Calendars.ReadWrite), but I’m not sure what is causing the issue.`

POST https://graph.microsoft.com/v1.0/subscriptions

{
  "changeType": "created",
  "notificationUrl" : "{myDomain}/api/comm/v1/Room/get-notify",
  "resource": "/users/{myUserId}/events",
  "expirationDateTime": "2024-09-13T18:23:45.9356913Z",
  "clientState": "a1b2c3d4e5"
}

Response:

{
  "error": {
      "code": "ValidationError",
      "message": "Subscription validation request failed. Notification endpoint must respond with 200 OK to validation request.",
      "innerError": {
          "date": "2024-09-12T02:39:53",
          "request-id": "1f37444a-9b9a-43b1-bad1-996bd52c8b15",
          "client-request-id": "1f37444a-9b9a-43b1-bad1-996bd52c8b15"
      }
  }
}

Server Code:

[HttpPost("v1/Room/get-notify")]
public IActionResult Index([FromBody] JsonElement requestBody)
{
    if (Request.Headers.ContainsKey("validationToken"))
    {
        var token = Request.Headers["validationToken"].ToString();
        return Ok(token);
    }

    string jsonString = JsonSerializer.Serialize(requestBody);
    var query = "INSERT INTO [Room].[JsonContent] (JsonContent) VALUES (@JsonContent)";
    _connection.Execute(query, new { JsonContent = jsonString });

    return Ok();
}