Example.cs

using UnityEditor;
using UnityEditor.Toolbars;
using UnityEngine;

public static class Example
{
    [MainToolbarElement( "ピカチュウ", defaultDockPosition = MainToolbarDockPosition.Middle )]
    public static MainToolbarElement Create()
    {
        var image   = ( Texture2D )EditorGUIUtility.IconContent( "SettingsIcon" ).image;
        var content = new MainToolbarContent( image );

        return new MainToolbarButton
        (
            content: content,
            action: () => Debug.Log( "ピカチュウ" )
        );
    }
}