Select the client for which you would like to add the description from the 'Clients' tab from the bar at the left of the screen.
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Click on 'Edit' at the top right corner of the screen. A popup will appear.
At 'Descriptions' category choose the language of the description you would like to enter.
After selecting the language, a tab in which you can write the description will appear. Select 'Update client' at the right bottom corner of the popup to save the changes.