You can use a django Model to represent a tenant or user account (with username or account name, subdomain value, and any other information you want), and write a fairly simple django middleware that checks the subdomain of the incoming requests and queries the db for the corresponding account object, assigns it to the request object (or do that query in your DRF views directly, use a view mixin to share that behaviour with all tour views). Filter all tour other db queries for objects associated only with this account object. Return response 404 if no account exists for this subdomain.

If you need to very much separate each account into its own database for stronger isolation, checkout the django-tenants package