An extension is uniquely identified by its publisher and extension IDs. If you select the TODO Highlight extension, you will see the Extension details page, where you can find the extension ID, in this case, wayou.vscode-todo-highlight. Knowing the extension ID can be helpful if there are several similarly named extensions.
In a multi-root workspace, the command will open your .code-workspace file where you can list extensions under extensions.recommendations. You can still add extension recommendations to individual folders in a multi-root workspace by using the Extensions: Configure Recommended Extensions (Workspace Folder) command.
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