Take a look into this thread.
Choice 1:
You can use an API like Spy++ to access other windows controls.
Choice 2:
you should use Windows APIs like EnumWindow or FindWindow then use EnumChildWindows API to find the controls in the target window, like the textbox you are looking for, then the API SetWindowText
have a look here for some ideas: Why is EnumChildWindows skipping children? also search for these APIs names here in Stack Overflow and you will find many examples...
Choice 3:
I found this api which may allow you to complete your task : http://www.programmersheaven.com/download/56171/download.aspx