Fire onselectedindexchanged on radiobuttonlist

I have a radionbuttonlist with 4 options, but none are selected as default (by design).

How do I trigger an event when a users selects one for the first time? My onselectedindexCHANGED works fine.



Then how about using something like

if (IsPostBack) {}

in Page_Load()

it is already on the radio buttons.

What if you set autopostback to true?