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How to print alert message on the desktop once logged on to the desk page?

I am trying to print a message on the desktop when the user has logged in without modifying the core, ie from a separate app, could it be done?

You can achieve that by creating a Note.
Goto:

Tools > Note > New

If you want it to be seen by every user, you can make it public.
There is also option to Notify users with a popup when they log in.

No, that is, when you log in make a frappe.call and according to the request show a msgprint.

I am looking for the same functionality as well