Any customized properties can be defined in the 'data-options' attribute.
<input id="t1" name="n1" class="easyui-textbox" data-options="
required: true,
p1: 'v1',
//...
">
You can call 'options' method to get your defined properties.
var opts = $('#t1').textbox('options');
console.log(opts.p1);