<form action="?" method="get" name="chat" onSubmit="TextSchreiben()" style="font-family: Arial, Helvetica, sans-serif; font-size: 14pt;"> 
<input type="text" name="nachricht" id="nachricht" onKeyPress="if(this.keyCode == 13) TextSchreiben()" style="width: 330px; margin-left: -22px;">
</form>

function TextSchreiben()
{
	var $http,
		$self = arguments.callee,
		$nachricht = encodeURIComponent(document.getElementById('nachricht').value);
	if (window.XMLHttpRequest) {
		$http = new XMLHttpRequest();
	} else if (window.ActiveXObject) {
	try {
		$http = new ActiveXObject('Msxml2.XMLHTTP');
	} catch(e) {
		$http = new ActiveXObject('Microsoft.XMLHTTP');
	}
	}
	if ($http) {
			$http.open('GET', 'chat-eintrag.php?nachricht='+$nachricht+'&random='+Math.random()+Math.random()+'&PHPSESSID='+$session_id, true);
			$http.send(null);
	}
}