Maybe you didn't notice that this is a Word forum, not an Excel forum!
FWIW, In Excel, you could use either:
.. a custom number format, as in 00\,00\,00\,000 or #\,00\,00\,000
.. a text formula, as in =TEXT(100000000,"00\,00\,00\,000") or
=TEXT(100000000,"#\,00\,00\,000")