Call Function when textbox is active

I have a text box and I want the function to run only if the text box is active. (user has clicked or tab over to the TxtBox1) and the user presses F9 on their keyboard



document.onkeyup = handleKeyUp;

function handleKeyUp(e)
{
if(!e)
var e = window.event;

key = e.keyCode;
Frm.txtBox1.value = key;

if (key == 120)
{
alert("You pressed F9 key");
}

}

Put the onkeyup handler on the text box, not the document.