To send a message you need to click on the pencil icon and then search for a contact or enter a phone number directly in common US format, eg 6505678811.
When you receive a message, two things will happen. You'll get a browser notification and then you'll also see a red dot when you land on messages list.
For each message, you can use emojis, attach files and send messages by pressing "Enter" on your keyboard.
You'll also be able to attach to messages the following types of files:
- Photos: png, jpg, gif
- Files: pdf, msword, ms-excel, ms-powerpoint, zip
- Text: calendar, csv