Integration with TestRail test case management software allows you to link test results in TestRails to YouTrack issues. For more information about integration and settings, please refer to the YouTrack Test Management with TestRail page at the Gurock Software web site.
On the YouTrack side, the only thing that has to be done is enabling REST API:
In YouTrack web UI open Administration > Settings section
Select REST API checkbox in the System pane:
All other integration settings should be made on the TestRails side.