additional_plugins/serendipity_event_typesetbuttons/buttons/CustomButton.php

<?php
require_once 'Button.php';

/**
 * Class CustomButton
 */
class CustomButton extends Button
{
    /**
     * @var string
     */
    protected $open;

    /**
     * @var string
     */
    protected $close;

    /**
     * @return string
     */
    public function getClose()
    {
        return $this->close;
    }

    /**
     * @param string $close
     */
    public function setClose($close)
    {
        $this->close = $close;
    }

    /**
     * @return string
     */
    public function getOpen()
    {
        return $this->open;
    }

    /**
     * @param string $open
     */
    public function setOpen($open)
    {
        $this->open = $open;
    }

    /**
     * @return string
     */
    public function render()
    {
        if ($this->isLegacyMode()) {
            $this->addClass('serendipityPrettyButton');
            $this->addClass('input_button');
            $this->setOnClickEvent(
                "wrapSelection(document.forms['serendipityEntry']['" . $this->getTextarea() . "'], '" . $this->getOpen(
                ) . "', '" . $this->getClose() . "')"
            );
        } else {
            $this->addClass('wrap_selection');
            $this->setOpenTag($this->getOpen());
            $this->setCloseTag($this->getClose());
        }
        return parent::render();
    }
}