If you already have an existing channel in Microsoft Teams, you can actually convert it into a universal channel and start inviting external contacts to chat with you.
Note: The Universal Channels app will not work on private channels, only public ones.
At the bottom of your existing channel, find the "· · ·" and select Mio Universal Channels.
Click on Convert and invite.
Congratulations! Your existing channel is now a universal channel.
If you're having trouble seeing the window, you may want to check if the Microsoft Teams app was correctly installed to your team first.