That's the correct format, yes. It's not designed to be user-friendly, so you have to provide GUIDs, JSONs and what not.
If the users in question are valid Exchange recipients, you can use the friendlier Set-User cmdlet, as follows:
Set-User user1 -Manager user2
You can use the UPN, alias, even display name to designate the users, but again, they need to be valid Exchange recipients.