I am trying to setup friendly URL for SP Online site collection. For Example by default all SP Online site collections looks like  **http://companyname.sharepoint.com/sites/test**  
 
Here I don't want to have sharepoint.com in the URL instead I'm trying to setup something very friendly URL based on the projectname or business unit such as **https://projectname.companyname.com/test**

I don't have much experience on DNS setup. Can anyone please throw some light on this query that would be helpful. 
Also is there any limitations/cons if I setup friendly URL like this **https://projectname.companyname.com/test**


Thanks,
Kiran