How to add a Steam Game as a custom game to Wtfast
Log into your steam account
Go to Library
Right click the game you want to add
Select Local Files Tab
Select Browse Local Files.
This will give you the path to the steam game folder for the game.
Select game folder
then select the bin folder (this is where the exe file will most likely be. - some may have 32 or 64 bit options)
Then open WTfast.
Create your custom game.
Browse to the file you found via browsing steam.
Choose 'NO launcher'
Do not add a launcher parameter.
Run Wtfast - the game should now launch correctly.