How to send a message to a specific channel

Well, if you have the "client" instance of Discord.Client(), then use this:

SOLUTION UPDATED to work with newer discordjs versions

client.channels.cache.get(`channelID`).send(`Text`)

Don't forget the channeID parameter is a string type, not number. It's really simple yet precise.

Assuming you have the client (which would be an instance of Discord.Client) try finding the desired channel by using Client.find:

const channel = client.channels.cache.find(channel => channel.name === channelName)
channel.send(message)

If you don't have the client directly but have a message instance, you could always access it from within the Message.client property.