Since Slack webhook endpoints must be hidden, this can't be done directly from the browser without revealing that URL and inviting spammers to flood the Slack channels.

The application was not running a traditional backend, but rather being served statically and talking to Firebase via its API to store and retrieve data.

The solution was to write the messages to Firebase and create a 'postman' application in NodeJS, which periodically checked the database for unread messages, sends them to the appropriate Slack channel, and marks them read.