Insert text in field using java

[cpu killer]

Just wondering how I can make buttons that will insert certain text into a form field? I'm pretty sure it's javascript but not sure exactly how it's done or how easy it is. If someone could shed the light, I'd like a button that someone can click and a dialog will show up asking for img url and than it will insert the

 [img=imgurl] 

into a form field, as IPB does.

Also would like a simpler version of just adding

 [b][/b]

, etc.

Any help is appreciated, thanks :)