...
<SCRIPT LANGUAGE="JAVASCRIPT">
function Verify()
{
if (document.frm_Test.F_Text.value == "")
{
alert("Please enter some text");
document.frm_Test.F_Text.focus();
return false;
}
return true;
}
</SCRIPT>
</HEAD>
<BODY>
<FORM ACTION="somepage" NAME="frm_Test" METHOD="POST" onSubmit="return Verify();">
<INPUT TYPE="INPUT" NAME="F_Text" SIZE=20></INPUT>
<INPUT TYPE="SUBMIT" VALUE="Submit">
</FORM>
...