After completing missions (can be found on right side), another button will appear (yellow star/spark icon). After pressing that button a popup will appear with the code and it's description. After the missions reset you can redo them again to receive another code.
I recommend making a screenshot or writing the codes down, since the seen ones aren't stored for later view.
"nice connection betwen those 2 games" thnx :)
Cheers,
~NK