I am also curious about this subject, well infact I am new with javascript also. I tried what you said , putting inventory[1] or 0, and literally put a string number. like trying to send any item without definite name.
var offer = manager.createOffer("https://steamcommunity.com/tradeoffer/new/?partner=291070862&token=tWXn6IyJ");
offer.addMyItems(inventory[1]);
offer.setMessage("Here, have some items!");