If you are on Silver or Golden plan and want to set up a custom domain, you will need to set up a CNAME DNS record for "domain_you_want" pointing to "cdns.use1.cloud.rocket.chat" (for US region) and to "cdns.euc1.cloud.rocket.chat" (for EU region) - once done, let us know via submitting a ticket here on our Helpdesk or dropping us an email to support@rocket.chat and our Cloud engineers will configure your instance accordingly.