Integration is available both for Jira Cloud and Jira Server through your company settings interface. Clicking the "Connect" button next to the Atlassian Jira integration you will see a popup with the options shown below. Choose either to connect directly with your Jira Cloud service or provide the required credentials to connect with your installed Jira Server.

Jira integration works in combination with a Slack or Microsoft Teams integration and allows you to resolve ticket summaries by referencing them through ticket keys in planning poker sessions.

For example, you can estimate 

/planpoker ACME-344 

in Slack or 

@Team O'clock pp ACME-344 

in Microsoft Teams and the integration will automatically extract the summary of the ticket and display a more verbose version with a link to the actual ticket in Jira.

To enable the integration you need to visit the organization settings page and click the "Connect" button in the Jira integration space. The integration works only within Jira Cloud accounts.