How to set default body message for Privatemsg embedded on page?
Have a look at the descriptor of the privatemsg_new function in privatemsg.pages.inc:

privatemsg_new($form, &amp;$form_state, $recipients = '', $subject = '')


It doesn't take a $body argument so you will need to alter the form once it has been created.


Make a custom module
Use hook_form_FORM_ID_alter
Set $form_state['values']['body'] to your desired value