You can head over to the licensing portal (my.atlassian.com) and click on New Evaluation License (or click this link) and select Portfolio from the dropdown. Then you'll need to copy/paste the license in the Manage Add-Ons page in JIRA.
You've tagged this question with server so this next bit doesn't apply to you, but just for the Cloud folks who see this from searching - Cloud extensions need to be done by contacting support. More details are here.
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