Something to try, although it will be a bit convoluted.
A) In the WinForm app tie the button click to a call to a WatiN procedure.
B) In the WatiN procedure
- Attach to the open IE browser
- Have a javascript method that finds the Textbox that has focus and gets the value of that textbox. Another approach would be to use WatiN to get all the textfields then loop through them all looking to see which has focus, and get the value that way.
- Return the text value as the return value to the WinForm app.
In theory, I believe all this to be possible though I have no specific experience tying a WinForm app to call a WatiN procedure.