如何手动将POST数据输入浏览器

I'm debugging my php script and need to try to post data to by manually inputting it into the URL in my browser.

The javascript which sends the request is below. How do I enter the correct data into my browser so it's encoded in the same way as the javascript function? I tried encoding the string with http://meyerweb.com/eric/tools/dencoder/ and putting sendmail.php?q="the encoded string"... but that didn't work. Do I have to add more information?

function SendPHP(str, callback){
    xmlhttp = new XMLHttpRequest();  
    str = "q=" + encodeURIComponent(str);

    xmlhttp.open("POST","sendmail.php", true);
    xmlhttp.onreadystatechange=function(){
        if (xmlhttp.readyState == 4){
                      inProgress=false;
            if(xmlhttp.status == 200){
                            callback(xmlhttp.responseText);
            }
        }
    };
    xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");  
        if (inProgress==false){
        inProgress=true;
        xmlhttp.send(str);
        }
        else{
            writeDisplayConsole("ERROR: xmlhttp fired twice!");
        }
}

You may want to look at Fiddler - the web debugging proxy. https://stackoverflow.com/questions/tagged/fiddler

One of Fiddler's features is Composer, which lets you edit a previously seen request, or create a new one from scratch. Check out e.g. this answer, it deals with a very similar issue: check POST request with Fiddler