Press Cmd + O on macOS or Ctrl + O on Windows / Linux to activate the open dialog, select the folder and click "Open."
Note. This answer was written before VS Code implemented several workspaces of several folders. If you open the folders in this way, it will replace all your workspace with the folder that you selected to open. If you want to add a folder to an existing workspace, you must use Cmd + K , Cmd + O on macOS or Ctrl + K , Ctrl + O on Windows / Linux.
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