Find the DefaultInput.ini file, located in your Steam folder, for Windows users this is normally located at
Program Files (x86)\Steam\steamapps\common\Mission 202\RxGame\Config
Go into the file and search for "primary default bindings". You want to put this command .Bindings=(Name="O",Command="toggleping | toggle ping") on a new line, then save the file.
Adding this binding will reset all of your other bindings in game, and you'll have to redo them.
The key can be changed, but make sure it's not already in use for another binding!

In game, try pressing your keybinding once you've gotten past the character select screen. It doesn't seem to work 100% of the time - you should see your ping in red text on the left side of your screen.
This command is very useful for telling which servers you're on.
WARNING: If you spam the command too quickly, it can crash your game.