Typecho background custom field multi-select code is as follows. Perform research. The multi-select is the value corresponding to the option name. The default is the first one. It mainly depends on the subscript.
$biaosi = new Typecho_Widget_Helper_Form_Element_Select('biaosi',array('1'=>'空位','2'=>'原创','3'=>'转载','4'=>'实用','5'=>'图片','6'=>'视频','7'=>'音乐','8'=>'种草'),'1','醒目标识,这文章更有个性','默认为不显示');
$layout->addItem($biaosi);