How to make a greasemonkey script to automatically set seacrh field value to particular value?

This is my code, which is not working:

var id=document.getElementById["thesearchbox"].value;

if(id==="") {

if(confirm("wanna search for fifa 16 crack?"))
{
document.getElementById["thesearchbox"].value="fifa 16 crack";
}

}

Here thesearchbox is the id of search field.

replace ["thesearchbox"] by ("thesearchbox")

thanks it worked.