Adding Text To Textarea - Create An Additional Button That Will Insert Some Text - Certain Html Tags

I am creating a small CMS module for a client. I created a little form and when they click Submit, it goes straight out into an include (.inc) file, which is connected to the web page to be displayed.

The trouble I am having is that I'd like to create an additional button that will insert some text (certain html tags to make their life easier, etc) - I got it to work, actually. The script executes and the text is inserted - but once the script runs and the page refreshes (or whatever it does), the text then disappears. The only way I can seem to get it to stay put is when I use "onmouseup" instead of "onclick" - which means that every time the user accidentally mouses over the thing, it inserts the text.