You can send a direct message to anyone in your organisation or any external contact if your organisation has enabled this feature in the Google Workspace admin console.
Click in the text box along the bottom of the chat screen to type a message.
By default message history will be kept - to turn off the history for the conversation, click on the drop-down arrow to the right of the name of the person you are chatting to and choose Turn off history.
Click on Send or press Return to send a message.
To add italic to a Hangouts Chat message, add underscores around the text e.g. _important_
To add bold to a message, add asterisks e.g. *Very important*
Add strikeout using the tilde e.g. ~remove~
Google Chat shows the out of office status people set up in Google Calendar. If someone is away from the office, Chat shows their out of office status when you open a chat with them. You can still send the person a message, but they might not reply until they're back.
Click on the information icon to view more out of office information: