Adding values from a list of checkboxes to a textbox

I am new to jQuery, so I am not familliar with syntax of it.

The problem I am facing right now is, I have a dropdown list with some values and corresponding checkboxes. When someone selects a particular value from the dropdown i.e checks a checkbox from dropdown, I want to add the 'value' of the checkbox to the textbox which is besides the dropdown of checkboxes.

The code I have come up with is

$(".productr-dropdown li input[type='checkbox']").click(function(e){
        var cmd = $(this).val();
        addToTextBox(cmd);
});  

function addToTextBox(cmd){
    command = $(".z:eq(2)").find("#platform_Command").val();

    if (command.indexOf(' '+cmd+' ')>=0)
        return;
    else {
        // THIS IS THE NEXT PART WHERE I AM STUCK
        // I don't know how to add/replace the contents of 'cmd' 
        //  to the contents of 'command'(which is the value of the textbox)    
    }       
}

To better explain my problem I will write a use case here, The user checks a checkbox from the dropdown, e.g the value of the checkbox checked is 'abc', the textbox in which I want to add this 'abc' already has a value say 'xyz hijkl defg'. what should happen is the value'abc' should get added to the textbox in such a way that the value of the textbox would be'xyz abc hijkl defg'

The problem I have is I am not sure whether I am invoking the correct event also.

One more approach I have in mind is, $(document).click(function(e) { $(".productr-dropdown").hide(); $(".productr-dropdown").each(["input='checkbox':checked"], function (e){ addToTextBox($(this).val()); }); });

Which one is better ?

What is the syntax to only select values in the dropdown which are checked ?

A HTML from you would be kind. I improvized here:

JSFIDDLE demo

I'm still not clear about what are you trying to achieve btw.

can't you do something like so:

$('#myTxt').val( $('#mytxt').val() + cmd )?

You will want to rewrite from

command = $(".z:eq(2)").find("#platform_Command").val();

to

command = $(".z:eq(2)").find("#platform_Command");

and from

if (command.indexOf(' 'cmd+' ')>=0)

to

if (command.val().indexOf(' 'cmd+' ')>=0)

so that you can easily just use the following to adjust the value

command.val(command.val() + " " + cmd);