Add link to send a private message to node authors on content types

To add a link to send a private message to the author of a node, visit Admin -> Structure -> Content Types (/admin/structure/types), and for the content type you want to add the link to, select 'Manage Display'. Next, click the view mode you want to add the link to, and drag the 'Private message thread link' element out of the 'disabled' fields to the location you want.