How to force the '.on("input")' method via script?

Question

I suppose my method:

$(document).on("input", "#textbox", function(){
    alert("oi");
});

What script can I force the Alert call to ("hi") ?

I tried a lot and nothing:

$("input#textbox").val("1");
$("#textbox").val("1").on("input");
$("#textbox").input("1");
$("#textbox").input("1").change();

http://jsfiddle.net/Lyqu4s92/

Expected behaviour:

When entering a character in the quantity field, a function must be called that calculates the value and quantity fields to play in the total field. When starting the page this function has to be called as well. The real case is this: a field with value, another with quantity and another with empty total. When opening the page the quantity field will be by default value 1, the value field will vary and the total will be calculated on top of these two values.

Answer 1

You came close in the fiddle:

$("#textbox").trigger("input");

http://jsfiddle.net/Lyqu4s92/1/

Answer 2

If the goal is simply to call the function, why not give it a name and call it directly?

function callback(){
    alert("oi");
}
$(document).on("input", "#textbox", callback);
callback();