Create a DKIM key to provide to a 3rd party so we can send as them?

We use o365/Exchange Online. All our SPF and DKIM configs are good for our domain (including 3rd party senders).

Say we have a partner who has domain XYZ123.com (with their own mail servers) and they want us to be able to send as them (from our o365 Exchange).

So this is the opposite of what I've done in DKIM in the past, and can find no info on doing it in reverse.

How do I generate/obtain the 3rd-party DKIM signature (and selector) for our mail server, to provide them for the TXT record (for their DNS) to authorize us to spoof XYZ123.COM?