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You can download and install DocuSign just like any other app for Jira, via built-in Atlassian Marketplace. To do this please follow those steps:
Log into your Jira instance as an admin
Click the admin dropdown and choose Manage apps
Click Find new add-ons from the left-hand side of the page
Locate DocuSign for Jira via search
Click Try free to begin a new trial or Buy now to purchase new license for DocuSign for Jira
Enter your information and click Generate license when redirected to MyAtlassian
Click Apply license
After installation you are ready for the next step - you need to go through Jira initial setup