Send Cross Domain HTTP Post With HTML Forms

I have this HTML form that I need to do some processing before submitting:

Solution 1:

You want to use the form's onsubmit.

I changed your html (and especially inline javascript) a bit (for clarity):

<form action="http://appid.appspot.com/api/foo" id="contact-form" method="post"
name="contact-form" onsubmit="return doStuff() ? true:false;">
    <fieldset>
        <input name="email" value="Email"
             onblur="if(this.value===''){this.value=this.defaultValue}" 
            onfocus="if(this.value===this.defaultValue){this.value=''}" > <br>
        <input name="subject" value="Subject"
             onblur="if(this.value===''){this.value=this.defaultValue}" 
            onfocus="if(this.value===this.defaultValue){this.value=''}" > <br>
        <textarea name="message" 
                onblur="if(this.value===''){this.value=this.defaultValue}" 
               onfocus="if(this.value===this.defaultValue){this.value=''}"
                      >Message</textarea>
        <div class="buttons">
            <a href="#" onclick=
            "document.getElementById('contact-form').reset()">Clear</a>
            <a href="#" onclick=
            "if(doStuff()){document.getElementById('contact-form').submit();}">Send</a>
        </div>
    </fieldset>
</form>

As you can see the autofill/autoclear on the input-elements now kind of repeats, so you could factor it out in a separate universal function.

Here is the javascript function that runs before the form actually submits:

function doStuff(){
    var eml=document.getElementsByName('email')[0].value;
        msg=document.getElementsByName('message')[0];
    msg.value = 'Email: ' + eml + ' Message: ' + msg.value;
    alert ('message has been submitted');
    return true;
}

Working JSFiddle here

Doing this cross-domain might get tricky if the other domain uses session-cookies and/or checks document referrer.

Update: I see you updated your question and now want to check and display the server-response in your message to?