Create a dedicated WordPress user for the agent with the Author or Editor role.
Signed in as that user, go to Users, then Profile, then Application Passwords, and add one. This needs WordPress 5.6 or later and HTTPS.
On the agent's WordPress card, enter the site address with https:// (include a subfolder such as /blog if WordPress lives there), the username and the application password. Spaces in the password don't matter.
401: the username or application password is wrong, or a security plugin has turned off application passwords.
404 at /wp-json/: permalinks are set to Plain. Choose any other option under Settings, then Permalinks.
A firewall or Cloudflare blocks posting: allow requests to /wp-json/wp/v2/.
More: WordPress integration.