Make sure you implement the yahoo and connection libraries.
You can download these from the YUI developer network.

<script type="text/javascript" src="static/js/yui/yahoo-min.js"></script>
<script type="text/javascript" src="static/js/yui/connection-min.js"></script>

The script:

<script type="text/javascript">
      var request;
      function doSomeRequest(servletName, servletArguments){
          var arg = "myParam=" + getPopupContent(servletArguments).replace(/[\n\r\%]/g,''); //I replaced linebreaks and % which breaks requests.
          YAHOO.util.Connect.asyncRequest('POST', servletName, callback, arg); //note the arg is only for POST methods
          	var callback =
			{
			  success: function(o) {
			  	//do something with the response
			  },
			  failure: function(o) {
			  	//do something with the error
			  }
			}
		}
</script>

Download the files or see more examples on YUI Developer network.