Grafana webhook alerts to App Engine (then to Webex)

I am trying to send alerts from Grafana to Webex teams, but as I understand it this feature is not a priority for the Grafana team. As such, I made a REST API app on App Engine (GCP) which I thought could work as a proxy between Grafana and Webex. However, when I try to send the alert to my endpoint(s), it logs a 405 error message (Method Not Allowed). I was wondering if anybody else had any experience with using Webhooks, and could possibly point me in the correct direction?

I have tested the endpoint(s) with Postman, so there should not be anything wrong there.

PS. What is the username and password fields in the Grafana alerts Webhook schema?

Update: When I specify the methods parameter for the Flask app I am able to receive the data through the request. However, I do log a 500 error in Stackdriver, which seems pretty weird: get an error message, but still get the data?