flutter-quill/doc/configurations/custom_buttons.md

Custom QuillToolbar Buttons

You may add custom buttons to the end of the toolbar, via the customButtons option, which is a List of QuillToolbarCustomButtonOptions.

To add an Icon, we should use a new QuillToolbarCustomButtonOptions class

    QuillToolbarCustomButtonOptions(
        icon: const Icon(Icons.ac_unit),
        tooltip: '',
        onPressed: () {},
        afterButtonPressed: () {},
      ),

Each QuillCustomButton is used as part of the customButtons option as follows:

QuillToolbar(
  configurations: QuillToolbarConfigurations(
    customButtons: [
      QuillToolbarCustomButtonOptions(
        icon: const Icon(Icons.ac_unit),
        onPressed: () {
          debugPrint('snowflake1');
        },
      ),
      QuillToolbarCustomButtonOptions(
        icon: const Icon(Icons.ac_unit),
        onPressed: () {
          debugPrint('snowflake2');
        },
      ),
      QuillToolbarCustomButtonOptions(
        icon: const Icon(Icons.ac_unit),
        onPressed: () {
          debugPrint('snowflake3');
        },
      ),
    ],
  ),
),