Are you referring to the Create a Message operation? https://developer.webex.com/docs/api/v1/messages/create-a-message
If so, if you send your example markup in the html key (or even the markdown key, but not the text key) of the POST body, it should be displayed as expected:
POST https://webexapis.com/v1/messages
{
"roomId": "Y2lzY29zcGFyazovL3VredactedzNjRkMGUx",
"html": "this is <b>bold</b> text"
}