One way that works is to add the word site with a colon punctuation in front of the site url, followed by a space, followed by your search parameters:

site:

then add

https://forum.lowcarber.org/

then add a space then your search

kefir

examples:

this link may work: https://www.bing.com/search?q=site%...27CA7E214EB3526

but a picture is worth a thousand words:

https://i.imgur.com/GrTpAmp.jpg?1