...
- Click the Url https://marketplace.atlassian.com to navigate to Atlassian Marketplace.
- Search for the app that needs to be applied with an Evaluation license.
- Go to Versions and select See all versions.
- Select the version of the app which is compatible with Server (or) Datacenter and Click Try.
- Provide your Organisation name and click on Click Generate License.
- Copy the generated Evaluation License.
- Open the browser and type <Url of your instance>/plugins/servlet/upm.
- Select the App. Go to the License key, click on the pencil icon and paste the Evaluation license.
- Click Update.
...