function comboChange() {
 var options = {
 success : showInitResponse,
 url : "/path/test.action",
 contentType : "application/x-www-form-urlencoded;charset=UTF-8",
 type : "post", /* get, post */
 dataType : "html" /* xml, html, script, json */
 };
 $('#INIT').ajaxSubmit(options);
 }
// post-submit callback
 function showInitResponse(responseText, statusText) {
 //submit후 작업부분
 document.getElementById('result').innerHTML = responseText;
 }
<s:form name="INIT" action="INIT" theme="simple" method="post">
 <s:select id="apiId" name="apiId" list="#{'INBO01':'INBO01', 'INTC02':'INTC02'}" onchange="javascript:comboChange();"></s:select>
</s:form>
<div id="result"/>