You would have to install a global keyboard hook, track the active window to know when input goes to a different application or input control, and simulate key strokes upon auto completition.
How to do that can be found out with a simple web research:
SetWindowsHookEx function (Windows)
SendInput function (Windows)
But have you thought about possible side effects?
It is a security risk. Your service would also catch and store user names and passwords internally. For that reason I would not use it; especially when it provides an auto learning feature which stores entered strings not only temporarily.