Hey there,
currently iam struggled at redirect custom hostnames to a subdomain.
Example at my side:
busaku.de CNAME to an als loadbalancer (eu-region)
us.busaku.de CNAME to an loadbalancer in US region
Under Custom-Hostnames I add “busaku.de” as an “Fallback Origin”.
So, at my Custom-Hostname I will try to add “us.busaku.de” as a CNAME and going to US-Loadbalancer.
But I will always land on the eu-loadbalancer.
example customer domain:
superdefaultdomain.com → aaa record to customers server
saas.superdefaultdomain.com → CNAME to us.busaku.de → you will land on EU-Loadbalancer.
Anything I don’t understand? Isn’t it possible?
Will I need a new domain for different targets in custom-hostnames?
Thank you in Advance 
Patrick