How to autofill windows user password

system("runas /user:username myprogram.exe");
//------read user password file-----//

It won't work because when program proceeds to the system command process, it will stop to get to the reading process until user him/herself has finished the password, and that's not what I want.

So is it possible to read user password file and then automatically fill it into where users are expected to fill in themselves?

Thanks!:)