Subdomain for Microsoft Planner in Office 365

Hello, Currently our company has a subdomain (e.g. webmail.companyname.com) pointing to mail.office365.com. We did that by creating a CNAME record in the DNS manager of our domain name manager (GoDaddy). Now, we're trying to do the same with the new Microsoft Planner (its url is tasks.office.com). We tried to add a CNAME fecord (e.g. tasks) to that URL and it does not work (an error message from MS is displayed). We searched for this in forums and the web, and we did not find an answer. How can do it? Thanks