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Step 1: Installing AssertThat - BDD & Cucumber for Jira is straightforward. Go to Jira Settings → Apps then select find new apps. Alternatively click on the below link
Marketplace Atlassian AssertThat -BDD Test Automation in Jira
Step 2: Select install.
Step 3: Start configuring Enable & configure AssertThat Plugin and using https://assertthat.atlassian.net/wiki/spaces/ABTM/overview?homepageId=728826175