Insertion into textarea

I have a button next to a textarea when they click that button it launchs another window. Now I need another button that will take a value (it will be an html code) close that window, and put that value in the textarea.

Sort of like how when you click the link button in this wysiwyg editor it pops up a window that asks for some stuff and then inserts it here.

Any help is much appreciated,
thanks,
Jordan

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