Well, you can do that by adding the app that contains your client model to your SHARED_APPS settings. After this, you can create a user under the public schema using 'schema_context' as follows:
from tenant_schemas.utils import schema_context
with schema_context('public'):
# create user
You can also access your client model from your public schema by creating a superuser in your public schema. Try...
$ python manage.py tenant_command createsuperuser --schema=public
And login to your public tenant admin from which you have full access to your client model in public schema.
Note: before creating a user in your public schema, you have to first create the public schema using your client model as stated in https://django-tenants.readthedocs.io/en/latest/use.html#creating-a-tenant
Hint: The 'shell' command will help you in creating the schema (public).