It seems that many people want to have messaging platform-specific integrations, for example Issue #1365, and PR #1490. According to the documentation, this project is not accepting any new custom receivers. To remedy this, adding support for including a custom JSON body, similar to including a custom message template (as seen here) would give users the flexibility to add in whatever fields they need. This way, users don't have to run their own service to translate the body, and the maintainers don't have to maintain many custom receivers.