Adding attribute to a link

I am trying to add an extra attribute to the links dialog for a <a href=> tag, but the value seems to get lost?

Things I have changed are:
fck_lnks.js:
GetE('attToolTip').value = oLink.attToolTip ;
SetAttribute( oLink, 'attToolTip' , GetE('attToolTip').value ) ;

and fck_link.html
<span fckLang="DlgTooltip">Tooltip</span><input id="attToolTip" type="text" />

The field show fine; I can fill in a value and save it. But the next time I want to change the field it says "undefined".

Can anyone help me out, or point me in the right direction?

Thanks in advance
Edw!n


what I really want is that the user just fills in a value, like 'swell' and that the link gets an attribute like
<a href="www..." onmouseover="Tip('swell')" onmouseout="">