While in Finder...

• Press shiftcommandG to bring up Go to the folder:, then type in /private and press enter.

• Drag the tmp folder to Favorites in the Sidebar.

While in Finder...

• Press shiftcommandG to bring up Go to the folder:, then type in /private and press enter.

Now preform one or the other of the following actions:

• Select the tmp folder and press: controlcommandT

Or

• Drag the tmp folder to Favorites in the Sidebar.

The first method places the folder at the bottom of the existing Favorites, whereas the second method allows one to drag it where one wants. It also can be dragged after the fact to change its position in the Favorites.

Both methods have been tested by me to work in OS X Mountain Lion, OS X El Capitan, macOS High Sierra and I see no immediate reason one or the other will not work in other versions of OS X/macOS that actually support this action.