How to communicate with other software from my onscreen keyboard?

I need to make a C# or Java application somewhat like "windows onscreen keyboard". I am not sure how to fill the alphabets into other applications on clicking buttons on my form. How do I communicate with the focused textbox or textarea on any given windows software, browser etc.? Any hints or code examples please?

Solution

There is an informative video on youtube about this very subject:

http://www.youtube.com/watch?v=VgFXwWQwE9k

And this is the source: http://pastebin.com/drWY3qnz

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