Setup wildcard MX record (for Fastmail)

[moiety]

While setting up my DNS records for my email (hosted at Fastmail) I ran into an issue. Fastmail asks me to: “Email for subdomains of domain.tld is not currently handled by Fastmail. Remember to set MX records for *.domain.tld if you would like us to do so.” I’m happy to oblige but DirectAdmin tells me that an asterisk is an invalid value?

How do I add a wildcard MX record?