From the Settings page you can request a custom subdomain:
To add a custom subdomain, you can add the subdomain and save, or click the link for further instructions:
To configure this, you must set up a CNAME record on your domain to point from the custom subdomain to osmosis.apphb.com.
Once you submit the request, our support team will configure the product to accept the subdomain, which make take 24-48 hours.