Before enable the CNAME, you need to verify your root domain. To verify the domain go to Dashboard>Acount>Domain Verification.
Please follow the below mentioned steps to enable CNAME to the zone.
Login to StreamFast console.
Go to Zones.
Select VoD push or pull zone.
Click the Manage button nearest to your zone ID.
Click the “CDN Resource” option.
Scroll down the page and add your CNAME in the option “Custom CNAMEs“
NB: you must point your CNAME to our hostname before adding the CNAME(eg: vod.yourdomain.com -> push-1611.streamfast.io)