The onFocus event handler is fired when focus is made on that particular text box.
<html>
<head>
<script language="JavaScript">
<!--
var counter = 0;
function sendAway(){
document.myForm.myText2.focus();
document.myForm.counter.value = counter++;
}
-->
</script>
</head>
<body onLoad='sendAway()'>
<form name="myForm">
<input type=TEXT value="First Box" name="myText1" onFocus='sendAway()'>
<input type=TEXT value="Second Box" name="myText2"><br>
<input type=TEXT size=2 value="" name="counter">
</form>
</body>
</html>