Powershell: search for a string in command output

Hello,

I am trying to search for a string in an output command.
In Exchange, I am finding if I find a username in a result of Get-MailBoxPermission.

This is the command I run (but not work)
This command does not return me an error, but I know that I should find the string userB, because it is mentionned in the result list as DominName\userB.

I am sure it is possible, and that I am not using the correct command.
Can someone help me on this?