1). Validate with IT that your external contact has been added to our O365 tenant
2). Once the contact is added, open up Teams start a new chat
3). Find your contact by typing in the name (in this example I will find Nate Corley from CareOregon), select the name with (External) next to it, this is the correct profile to use in Teams chat.
Once you start a chat, you will see an External label next to their name: