Copy in clipboard in asp.net

Bad idea. If your purpose is to copy some text between windows, you need to do exactly that. Do you understand that Copy/Paste has the side effect, change of the clipboard content?
"Not by Javascript" is nonsense. If the data is changed on client size, ASP.NET knows nothing about it unless you postback this data, but why? By your description, your operation should be local to the client. Anyway, even if you want to send the data back to the server and handle it there and than update the parent window, I cannot see what is your problem.
—SA