setup friendly URL for SP Online site collection

Hello All

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 like this 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 https://projectname.companyname.com/test

Thanks,

Kiran