Passing control as parameter to javascript function

I am trying to pass a control's id to a javascript function that adds the value of it(the control which is a textbox) to a listbox but apparently I am not getting it right, can someone please correct me.

Thanks.

<input type="button" ID="btnAddtoLstBox" value="" title="Add this to the list" onclick="javascript:addToList(document.getElementById(btnAddtoLstBox));"
class="ui-icon ui-icon-refresh ui-corner-all" style="width: 20px; height: 20px; background-position: -64px 80px"  />

  // scripts to add list items
        function addToList(varTxtBox) {

                 // get the list box
                var lb = document.getElementById("uilstMemTypeTier");

                // get the text to add
                var toAdd = varTxtBox.value;

                if (toAdd == "") return false;

                // look for the delimiter string. if found, alert and do nothing
                if (toAdd.indexOf(delim) != -1) {
                    alert("The value to add to the list cannot contain the text \"" + delim + "\" as it is used as the delimiter string.");
                    return false;
                }

                // check if the value is already in the list box
                for (i = 0; i < lb.length; i++) {
                    if (toAdd == lb.options[i].value) {
                        alert("The text you tried to add is already in the list box.");
                        return false;
                    }
                }

                // add it to the hidden field
                document.getElementById("<%=uihdnlistBasedFieldsListItems.ClientID%>").value += toAdd + delim;

                // create an option and add it to the end of the listbox
                lb.options[lb.length] = new Option(toAdd, toAdd);

                // clear the textfield and focus it
                varTxtBox.value = "";
                varTxtBox.focus();
            }