If you have an existing Mattermost user account with the name circleci, the plugin will post using the circleci account but without a BOT tag.
To prevent this, either:
Convert the circleci user to a bot account by running
mattermost user convert circleci --bot in the Mattermost CLI.
If the user is an existing user account you want to preserve, change its username and restart the Mattermost server. Once restarted, the plugin will create a bot account with the name
To generate the keys needed, go to System Console > Plugins > CircleCI:
Generate a new value for Webhooks Secret. If the generated secret contains a forwardslash, please regenerate it.
Generate a new value for At Rest Encryption Key.
Go to System Console > Plugins > Management and choose Enable to enable the CircleCI plugin.
You're all set!