async javascript - onclick event gets data from server

[Steve Richter]

what topics, documentation am I looking for to do the following:
- when a button or anchor tag is clicked, I want to call back to the
web server to retrieve the complete url with which to open a new
browser window.

function WindowOpen( url )
{
window.open(
url, 'steve',

'location=no,titlebar=no,height=400,width=400,scrollbars=yes,resizable=yes') ;
}

function Button_OnClick( key )
{
url = GetFullUrlFromServer( key ) ; // how to async connect to
server ??
WindowOpen( url ) ;
}

<INPUT type="button" value="Button"
onclick="Button_OnClick( 'abc123');">

thanks,