Multitenancy is supported via setting the tenant assign on your conn. The easiest way to do this is to write a Plug. This is a well documented process elsewhere. A good example of what you may want to do is derive the tenant from the subdomain.

See the implementation from triplex for a good example. Extract your tenant name and use Plug.Conn.assign/3 to assign that value to tenant before the request is forwarded to your Ash API.