how to add an item to the editor toolbar?

Hi,

I’m writing a plugin to replace a placeholder like {placeholder} with some data from a database. It works fine so far if I add the placeholder in the post content, but now I want to add an icon or button to the editor toolbar to add this automatically when I click on the button.

I have tried to find it somewhere in the docs but I’m lost. Can anybody give me a hint where I can find a tutorial or documentation how to do that?

Thanks
Marian