How to notify specific Slack channels for a merge request

We just switched to Gitlab from Bitbucket, but having some teething troubles. One of the issues that I haven't been able to resolve is how to direct notifications about merge requests to specific channels.

We have a Slack channel per project and I would like to have the correct channel notified for each new merge requests, so that the members of the channel can know to review it.

How can this be done?