You really can't do it from a web app, if it was allowed then pop-up loving sites all over the internet would I'm sure be accounting for most of the worlds ink usage.
I had this issue when building a web app for a college, the most its possible to do is call up the print dialogue and have the user select the printer (best to have a decent print targeted CSS here):
javascript:window.print();Its not perfect but to be honest although they insisted on it when it was put in a year ago, emails and PDF's are much more widely used.