Came here to ask for the same. The current implementation for Forward Email is practically useless because the email you have as your login is unlikely to be one of the domains you’re managing. So even if you set a custom domain, literally every single email will be forwarded to your login email when you generate an alias since if no recipients are defined in the API request, it will default to your sign in email.
Or allow custom API requests as per: Custom API URL for self-hosted email alias services - Mobile Clients (most recent post, multiple links are not allowed in a reply)