I have found out how to add the 18+ version to Steam.
so you download the 18+ version to your files then tap on the download and it will create a folder that has everything that you need.
then you open the desktop version of requiescence and replace all of the files except for the Steam short cut, after you have replaced everything you should enter the game by double tapping the game and if you see a screen that appears and says "FYI 18+" parts you did this right . I am sorry for my poor grammar and spelling.