Currently is not possible to specify the developer portal domain from the APIManager CRD nor tenant CRD. Because there may be multiple tenants, all with their own developer portal, it would not be ideal to configure it in the API Manager CRD. Instead it would be better if this new feature is against the Tenant CRD[1].

In the latest version as of today (2.13) the developer portal domain change can only be done manually from the admin portal, which represents a limitation for customers who want to automate the provision of 3scale (including the developer domain) so they require that this can also be specified in the Tenant CRD.

[1] https://github.com/3scale/3scale-operator/blob/3scale-2.13-stable/doc/tenant-reference.md