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In the search field type "Email Task" (2), then hit enter. After a moment you will be presented with search results. Make sure that Email Task vendor is Atlassian Verified "Transition Technologies PSC".'
Directly after installation, and applying the license, to use the add-on will be ready to use. For for test purposes, after installation please click "Free Trial" button. This will generate trial license that you can apply for the add-on. After doing so, it will be ready to use.
When trial mode is present, "Buy now" (3) button will be displayed instead "Free Trial" one.