If you haven't already created your domain in heyflow, start here.
At Domain & SSL, navigate to DNS. On this page, clik Record hinzufügen.
Chose Typ: CNAME. The Hostname is your Subdomain. Zeigt auf is flow.heyflow.domains. TTL is 1 Stunde. Click on Speichern.
That's it. Now, you can go back to Heyflow and finish the Domain setup.