I'm using joker.com domain registrar and Office 365 needed to verify that I'm the owner of the domain. This is done by either adding TXT or MX record to the DNS records at joker.com (domain registrar).
TXT is the preferred method in order to minimize risks of causing issues in mail delivery if one manages to type in incorrect values for the MX record (in Finnish: ryssiä).
In my (test) scenario I wanted to register intra.jussipalo.com.
Office 365 instructs the following:
Now, in Joker.com, I only have the fields Host and Description:
Host field didn't accept @.
In Host field, type in the third level portion of the domain name, in my case intra. In Description, type in the MS=ms12312312.
Save and accept the changes and wait for some time (for me it took maybe 1-2 hours) and you can successfully verify the domain in O365 admin console.