As follows:

txtSearch: The text box ID
$("#txtSearch").keyup(function (event) {
    var keycode = event.which;
    if (keycode == 13) {
        alert(' You've hit enter ');    
    }
 });
 or
xObj.keyup(function(event){
     //Gets the key value of the current key
     //JQuery has a which property on the event object to get the key value of the keyboard key
     var keycode = event.which;
     //Handle the return case
     if(keycode==13){

    }
     //Handle the esc situation
     if(keycode == 27){

      
     }
 }); 
 or
$(document).keyup(function(event){
     //Gets the key value of the current key
     //JQuery has a which property on the event object to get the key value of the keyboard key
     var keycode = event.which;
     //Handle the return case
     if(keycode==13){

    }
     //Handle the esc situation
     if(keycode == 27){

      
     }
 });