There is no default shortcut, but you can define your own shortcut for the command. To do this, please follow the steps below:
1. Select the "Tools | Customize Keyboard..." main menu item.
2. In the Customize Keyboard window, select the Edit category.
3. Select the Comment Selection (or Uncomment Selection) command.
4. Set focus on the "Press new shortcut key:" edit box and press the needed key.
5. Click the Apply button to apply changes.