Keycode event in adobe javascript....

hello,
wanna capture keycode(i.e ASCII) value of keyboard ENTER in adobe forms javascript.I've written the code to call a web service on exit event of adobe JS
and its working for both keyboard ENTER as well as TAB.now i wanna disable tab functionality that's web service should get called only on Keyboard ENTER.
I tried out event.keycode but that doesn't exist in Forms Designer dropdowns options and if I use it anyway it doesn't work.
any help would be appreciated
 
Thanks.