Go to Spaces , click choose Contact a Person, and then select Contact a person on another instant messaging application.
Enter the person's instant messaging address, and click Contact person. Then, type and send your message.
Some of the options are different for these type of spaces:
Webex Teams | Send a Message to Someone on Another Instant Messaging App
You can send a plain text message to someone who is using Jabber, Pidgin, or another supported XMPP app. Your organization's administrator can enable instant messaging for specific domains or organizations.