@mcolvin There is not much information in upstream docs about this but from what I read in the threads below, the functionality you want is not there. There is a single client portal URL for the all clients to access.
https://forum.invoiceninja.com/t/client-portal-subdomain/11754 (seems to confirm that you cannot access clients by subdomain) https://github.com/invoiceninja/invoiceninja/issues/5079 (upstream feature request?) https://forum.invoiceninja.com/t/url-and-client-portal/15250 (unanswered)Maybe you can ask about this upstream again in one of the channels.