For Windows users

The Windows logo key can be used as the Command key.

For Linux users

Using the Command key in Xcode

Note that if you are experiencing issues using the Command key in Xcode, please see the following solutions: 

- Xcode Move/Resize UI-Elements in Storyboard Editor

- Alternative Method to “Ctrl+Drag” (“Control+Drag”) 

Sources: Microsoft Support's Windows keyboard for Mac map