First install Customize Toolbar via the plugin admin page.
b4755b54-85a1-45ed-b8e3-6561386dd29b-image.png
Once done, goto plugins->customize toolbar
4baf50e1-8847-4065-8db0-2392749c8080-image.png and check custom buttons
This will create the needed configuration file.
Goto plugins->open plugins folder... and move into the config directory from the newly created explorer window.
There you will find the CustomizeToolbar.btn
Open it and you will see examples how this needs to be done.
For example:
52d5099f-3115-4f8d-87eb-e645b8b4dfc4-image.png
More info on how this works is available via the CustomizeToolbar plugin menu.
The hammer symbol, on the far right in the menu, is used when no custom icon has been provided.