See the Project Settings - Notification Policies and Repository Settings - Notification Policies for creating, deleting and editing notification policies. 

Bitbucket has a complete event subsystem that fires events whenever anything happens inside the application. For example, a  event is fired whenever a repository  is created.

To notify Mattermost from Bitbucket when a repository is created, your Bitbucket Administrator will set up the correct policy.  Then, when a repository is created from Bitbucket, the designated Mattermost channel will receive a notification. 

Example:

A Bitbucket user creates a repository named XXX.  This project has, for its project channels, the Mattermost channel XXX.  With a "Repository Created" notification policy set for the project, the following notification would be seen in the XXX channel upon repository creation:

The date/time of creation are shown.  The notification shows the issue type, reporter, assignee, priority, status and resolution status. was created by the user  in JIRA. 

A link is provided to the issue in JIRA.

Here are the results in Mattermost for the different notifications: