Per guild prefixes are pretty cool! Here are some ways to get a prefix from a guild and such!

How to add a prefix to a specific guild?

handler.prefixes.set('guild id', 'new prefix')

How to get a prefix from a specific guild?

handler.prefixes.get('guild id') || handler.prefix

We recommend storing the prefixes in a db, and then, make a loop to add the prefix to the guild when the bot restarts.