See the Amplifier for Mattermost for JIRA Project Administration Guide for creating, deleting and editing notification policies.
JIRA has a complete event subsystem that fires events whenever anything happens inside the application. For example, an ISSUE_CREATED event is fired whenever an issue is created.
To notify Mattermost from JIRA when an issue is created, your JIRA Project Administrator will set up the correct policy. Then, when an issue is created from JIRA for that project, the designated Mattermost channel will receive a notification. Example:
A JIRA user creates an issue of type Task in the project HerzumAmplifierDemo (project key HQ). This project has, for its project channels, the Mattermost channel Amplifier Development. With an issue created event set for this setup, the following notification would be seen in the Amplifier Development channel:
The notification shows it was created by the user admin in JIRA. The date/time of creation are shown.
A link is provided to the issue in JIRA, and the table shows Reporter/Assignee/Priority/Status and Resolution.