How to add TXT record via hosting cPanel in Zone Editor?



I have a domain registered with a third-party registrar which points to my GoDaddy linux hosting via GoDaddy's DNS. I've recently signed up to MailGun which requires me to add additional TXT domain records. In most cPanels, this would be managed via the Zone Editor. However, GoDaddy's zone editor only allows me to add/edit/delete MX records. 


All documentation I have found are only relevant if the domain is registered at GoDaddy, which mine isn't. 


How can I add TXT domain records for a domain which isn't registered with GoDaddy, but uses GoDaddy's DNS for hosting?