How to set up DKIM for a subdomain in MailGun?

Question

Mail server

MailGun recommends sending mail from subdomains,
but when creating a subdomain, DKIM is missing in the admin panel, below are 2 examples: for a domain and a subdomain.
And this is not always the case - for some subdomains of norms, DKIM is created. I can't figure out what could be the reason.
How to be here? How does it work and what should be done?