Please try to download the updated panel plugin from
http://www.jeasyui.com/easyui/plugins/jquery.panel.js and include to the page. The 'method' property can be used to set the ajax method.
<div class="easyui-panel" style="width:800px;height:400px" data-options="
href: ...,
method: 'post',
queryParams: {...}
">
</div>