Normal browsers don't do POST operation, so either use a REST Client or save the following in an HTML file and open in a browser to create a workorder.
<form name="input" action="http://<server:port>/maxrest/rest/os/mxwo?_lid=username&_lpwd=password" method="POST">
Description <input name="DESCRIPTION" value="Description will go here" type="text"> <p/>
SITEID <input name="SITEID" value="BEDFORD" type="text"> <p/>
<input type="submit" value="Submit">
</form>