What have you tried?
Are you trying to open a new browser window automatically when a message is sent? You need to remember that, while you are accessing the node-red admin ui probably in the same browser, the flow is not running in the browser (front-end), it is running on the server (back-end). Two completely different contexts.
So to affect the front-end (browser) from the back-end (server), you need firstly to have a front-end app and secondly for that app to receive a message from the back-end. This is what Dashboard does.
However, Dashboard has its own limitations. It is a Single-page App (SPA). so you can't really open a "new window". But what you can do is use the Dashboard control node to switch to a different tab - assuming you already have the dashboard loaded.