Application to insert data in another application

Hi people,

I have to make an application, to insert data on another application (was not made by me).

The process is simple: Send values to textboxes, and simulate the Ok button.

I try to use sendkeys, and work ok. But the destination aplication will have the focus.

I research about SendMessage. How can I do this using SendMessage?

Exist another way to do this?

Thanks,

Felipe.