From a running instance navigate inside that directory and either press m{mark name} (e.g. md) or execute :bmark tag-name depending on whether you want to bookmark it with a character or a tag.
If you want to do it in vifmrc, put something like this:
" navigate to this via 'r
mark r ~/repos/
and/or this:
" navigate to it using :bmgo repos
bmark! ~/repos/ repos