enter code here

<object style="height : 1200px;width : 1300px;overflow:hidden;">
<form id="frm" action="http://XXX/?username=ZZZ_UserName&password=PWD&retUrl={/apex/response}" method="POST">

  //  <input type="hidden" id="username" value="ZZZ_UserName"/>  //Tis input tag not working, so directly passed Username and password in action only.
    // <input type="hidden" id="password" value="PWD"/>
    // <input type="Json" id="JSON" value="('{!fleet_Car_Set__c.JSON_String__c}')"/>---
</form>
</object>
<script>
alert("Inside Script");
alert('{!Fleet_Car_Set__c.JSON_String__c}');//---Here we are getting full JSON data, need to post this JSON to External System(http://XXX/).
    document.forms["frm"].submit();
</script>
</html>