put the content of 'predml.txt' in the predml field of a template named 'print', and the content of 'postdml.txt' in the postdml field.

Then open a content with an URL like this:

...openwebcms3.nsf/(ynDK_contentByKey)/MyContent??Open&template=print

then the content opens with the 'print' template which provides some information, then the plain content and opens the print dialog.