我发现您可以向按钮添加一个className属性，并通过文档选择按钮。getElementsByClassName查询选择器。所以代码应该是这样的

( function( wp ) {
    var MyCustomButton = function( props ) {
        return wp.element.createElement(
            wp.editor.RichTextToolbarButton, {
                icon: \'editor-code\',
                title: \'Sample output\',
                className: "foo"
                onClick: function() {
                    console.log( \'toggle format\' );
                },
            }
        );
    }
    wp.richText.registerFormatType(
        \'my-custom-format/sample-output\', {
            title: \'Sample output\',
            tagName: \'samp\',
            className: null,
            edit: MyCustomButton,
        }
    );
} )( window.wp );