I wrote another script that fixes some issues in the script posted by Ewwis:

- There was no way to close the dialog at the start.
- The second click action didn't work if Notification Center hadn't been shown after the last login.
- The script didn't work when there was a delay before the view for composing a tweet was shown. If it already contained some text, it wasn't cleared.
- The keystroke command only works for inserting characters that can be entered with the current input method.
- The Notification Center sidebar wasn't closed at the end.

It doesn't work when the Notification Center sidebar is open though.

	set answer to text returned of (display dialog "" default answer "")
	try
		set old to the clipboard as record
	end try
	try
		set text item delimiters to linefeed
		set the clipboard to paragraphs of answer as text
		tell application "System Events"
			tell process "Notification Center"
				click menu bar item 1 of menu bar 1
				try
					windows
				on error
					click menu bar item 1 of menu bar 1
					click menu bar item 1 of menu bar 1
				end try
				click button 1 of UI element 1 of row 2 of table 1 of scroll area 1 of window 1
				delay 0.1
				keystroke "av" using command down
				keystroke "d" using {shift down, command down}
				repeat 100 times
					try
						delay 0.1
						click menu bar item 1 of menu bar 1
						exit repeat
					end try
				end repeat
			end tell
		end tell
	end try
	try
		set the clipboard to old
	end try

It would be easier to just [use the API](http://apple.stackexchange.com/a/58344/3936).