Create a webhook

Available event types:
- subscriber.subscriber_activate
- subscriber.subscriber_unsubscribe
- subscriber.subscriber_bounce
- subscriber.subscriber_complain
- subscriber.form_subscribe, required parameter form_id [Integer]
- subscriber.course_subscribe, required parameter sequence_id [Integer]
- subscriber.course_complete, required parameter sequence_id [Integer]
- subscriber.link_click, required parameter initiator_value [String] as a link URL
- subscriber.product_purchase, required parameter product_id [Integer]
- subscriber.tag_add, required parameter tag_id [Integer]
- subscriber.tag_remove, required parameter tag_id [Integer]
- purchase.purchase_create