Add custom button

Unfortunately this is not the right place to ask. Our plugin does not provide any editor functionality, it just allows to use the visual editor for widgets.
For further info about adding custom buttons read the Codex documentation and this guide.

Oh sorry. And thx for reply.

Are you aware whether your plugin conflicts with trying to add a custom button?

I tried https://code.tutsplus.com/tutorials/guide-to-creating-your-own-wordpress-editor-buttons–wp-30182 and noticed it wasn’t working.

@one3rdnerd The article you linked is over 2 years old and in the meantime WordPress switched to the new version (4.0) of TinyMCE editor, so it’s possible that the code is no longer compatible.
Please see WordPress 3.9+ compatibility on Codex for further info.