As others have noted, WINDOWS+B for the system tray, then use the arrow keys to move through the items and either ENTER or SPACE or APPLICATIONS KEY - depending on what you want to do and how a particular application works.  NVDA responds to either but other programs do different things.  Note that if you have icons in the "overflow" area, you may consider disabling that and having all system tray icons visible all the time (otherwise you need to activate the overflow area and then navigate around that for any that aren't in the regular area of the system tray)