Point me to simple plugin as an example

In the same way WP has a “link” button, I want to copy that behavior. I want to paste in some text, and have the resulting text in the text area.

As a simple example, my button would be “SHOUT” and clicking it I could enter in ‘hi how are you’

And into WP text area it would put **hi how are you**

Not really what I want to do, but that would get me learning enough to start.