def METHOD_NAME(view_function):
    @wraps(view_function)
    def _handle_initial_validation_response(*args, **kwargs):
        """
        Handle initial validation of webhook endpoint.
        When subscription is created Microsoft Office365 servers
        immediately contact the endpoint provided POSTing to it with
        validationToken GET argument set. Endpoints are supposed
        to answer with 200 OK, text/plain MIME type and body set to
        validationToken. If an endpoint is unreachable or does not
        respond correctly subscription creation won't succeed.
        Subsequent POSTs contain change notifications.
        https://learn.microsoft.com/en-us/graph/webhooks#notification-endpoint-validation
        """
        validation_token = request.args.get("validationToken")
        if validation_token is not None:
            response = make_response(validation_token)
            response.mimetype = "text/plain"
            return response
        return view_function(*args, **kwargs)
    return _handle_initial_validation_response