Setting up dkim. Private key?

Question

Mail server

Hello.
Guys. There is a vest panel. For mail, you need to make dkim. Generated 2 keys. I specified public in dns. And where to insert private????
Somewhere it is written to copy it to a file and put it on the server. But I didn't really understand where.
I also read that through ssh commands. But also not very clear!
Can you elaborate on where to insert the private key?