function Dcheck(frm)
 {
  if ((event.which && event.which == 13) || (event.keyCode && event.keyCode == 13)) { frm.submit(); }
 }

function Fcheck(frm)
 {
  if (frm.Slink.value == '')
   {
    alert('Insert Site Link Please');
    return false;
   }
  frm.submit();
 }

---------------------------------------------------------------------------------------------------

    <form method="POST" id="MakeLink" name="MakeLink" action="MakeLink.php" onsubmit="return Fcheck(this)">
<input type="text" name="Slink" size="20" class="TextBox" style="width:130px" onKeyUp="Dcheck(this.form);" onKeyDown="Dcheck(this.form);" onChange="Dcheck(this.form);">
</form>