Make a popup and insert values from input to old site

[Gernot Frisch]

I've seen this in forums. You click on "smileys", there's a popup, you
click a smiley and it's in your text.

I have a textarea in edit.html. Then I click a button and want to have
a popup window, where I can edit another textarea. When I click a
button in the popup, I want the text from the popup's textarea to be
inserted into the edit.html's text area.

How can I do that?