input onclick new tab

try window.open

window.open('http://www.google.com');

update

live demo - http://jsfiddle.net/im4aLL/tzp4H/

function open_in_new_tab(url )
{
  var win=window.open(url, '_blank');
  win.focus();
}

Call that function when you want to open a link in a new tab. Also check here and here

Regarding your updated answer to your original post:

Adding ,'_blank' after the .value like so:

<input type="button" onClick="window.open(document.test.choose.options[document.test.choose.selectedIndex].value,'_blank');>

Actually opens in a new tab instead of an entirely new browser window.