<div align="center">
<select id="search_engine">
<option value="https://www.baidu.com/s?wd=%s">百度</option>
<option value="https://cn.bing.com/search?q=%s">必应</option>
<option value="https://www.sogou.com/tx?query=%s">搜搜</option>
<option value="https://www.so.com/s?q=%s">360搜索</option>
</select> 
<input type="text" id="search_word"> 
<button id="button_search">搜索</button>
</div>

button_search.addEventListener('click', function(){
   var url = search_engine.options[search_engine.selectedIndex].value.replace('%s', search_word.value);
   window.open(url, "_blank");
}, false);