Your Personal Room must be associated with Webex App before you can add your Personal Room link to a message. See Webex App | Set up your Personal Room for more information.

When writing a message, select My Personal Room invite.

Your Personal Room link is inserted in the message. You don't have to type it. Here's an example: https://company.webex.com/meet/acassidy