You don't--at least not without a lot of tedious work. The way this *should*
work is as follows:
1. You set a right tab stop short of the page numbers and give it a period
leader, removing the leader from the page number tab stop.
2. You then have to get Word to honor an extra tab character in the TOC
entry (since two tab characters will be required to reach the page number
tab stop). This is where the trouble begins.
3. You insert a tab character at the end of the heading in the document,
then add the \w switch to the TOC field, which is supposed to preserve tab
characters in TOC entries (and apparently does in some situations), but this
just doesn't work: the trailing tab character is ignored.
It may occur to you to insert an extra tab character in each TOC entry
manually, but every time you update the TOC, it will be removed. The only
thing that will work, therefore, is to wait until editing is complete and
you are confident that no page numbers will change, then either unlink the
TOC and add the tab characters or add the tab characters and lock the TOC.
It's really annoying that all the above is true, but it's a sad fact. Sorry.