Use a RAT or a stealer, you could even post a fake application of it, some people will know that hah its fake and think that the only way it hacks them is if they enter the correct information in the login place in the app. But instead you bind your RAT,Stealer or keylogger to it, and even if that doesnt work but you have infected him/her with a RAT then just threatn him/her to give you hes/her steam info or you will fuck up hes/her computer.
PS: This is the last time im helping on Black Hat issues, But if any of you need any White Hat help just pm me!