During the off-season the U21 or National Teams get new managers.
During that time you can choose to go for it here:
(http://www.buzzerbeater.com/country/8/ntelection.aspx)This is the example for Germany. For USA you'd need to replace the 8 in the URL with a 1. that's the country ID.
Following this, all managers can vote the NT/U21-Manager for their own country.
U21 elections take place after uneven season numbers (e.g. between season 15 and 16)
NT elections take place after even season numbers (e.g. between season 16 and 17)
I.e. every NT/U21-manager will be elected for 2 seasons.
We have both said a lot of things that you are going to regret.