How to add an article in Odoo message?

Turn on the setting 'Articles in Email Composers'

Create a new tag (e.g. 'odoo / helpdesk'). In the table 'Applied to' use a required model (e.g. 'helpdesk ticket') or use it for all documents (the check box 'Apply to all documents'). As advanced option you can apply filters: for example, to show articles only for 'new' tickets, not for 'in progress' ones

Assign this tag for required articles (for a few articles use the mass action 'Mass update')

Go to the discussion under instance of this document type, press the button 'Send message', open 'Full composer' (arrows in the bottom right corner)

Push the button 'knowsystem' in a message editor, select required article(s)

Choose an action: to update message body with article contents, to attach a PDF version, to share a public url