Create send user private message link in a node template content type
Assuming you want to create a link to the author, you should be able to create a simple link in the template
 - Field that links to a path/url defined with Tokens, is it possible?

It would follow something like: http://www.example.com/messages/new/[node:author]

You can see a list of tokens here: https://drupal.org/node/390482#token-node