How to set up DKIM?

[XFFA.IR Bot]

I received two keys, an open key and a private key, from the easydmarc service.

I need to add the open key as a DNS TXT record.

But where should I add the private key? In Xenforo or somewhere on the server?

"Private key You must enter this key in your DKIM signer. It must be kept secret, as anyone with access to it can stamp tokens pretending to be you" - I didn't understand this...
Please help me.

Thank you.