Why do I see
https://app.futuresimple.com in the URL of my Sell account?
Future Simple was the parent company of BaseCRM. When you create a Zendesk account, you must create a Zendesk subdomain. If you have a legacy Sell account, you might not have a subdomain unless you migrated your account.
While there are both legacy and non-legacy accounts, when you sign-in in your Zendesk subdomain and go to the Sell product, all of the requests to https://yoursubdomain.zendesk.com/sell redirect to
To find out your subdomain, see the article: How do I find my Zendesk Sell subdomain?