[ruslan_mart]

var checkbox = document.getElementsByName('urgent')[0];

document.addEventListener('keypress', function(e) {
    if(e.keyCode == 115) checkbox.checked = !checkbox.checked;
});

или

document.addEventListener('keypress', function(e) {
    if(e.keyCode == 115) document.getElementsByName('urgent')[0].checked ^= 1;
});