SpamAssassin - Change Index for Domain

So, when Junk Mail filter turned on in panel, how can I set the SpamAssassin index for an entire domain? I saw on the wiki how user could log into their webmail to change it, but that’s a lot of work for one administrator to have to do.

Can this be done from SSH with a couple commands?

Can someone point me to a how-to?

What is the filter index by default?