The Issue field in TestRail is used to link defects from JIRA. All links created from this field will appear in the external references section of the defect, giving you the ability to quickly access and resolve linked issues.
To link defects from JIRA:
Log in to TestRail and navigate to the Test Case containing the defect that needs to be linked to JIRA.
Select the ‘Edit’ button and navigate to the ‘External references’ section of the test case.
Enter the URL of the JIRA issue and select ‘Link’.
Repeat the same step for each defect that needs to be linked.
You can also refer to this guide which explains the process step-by-step: