The combo box isn't created until after the DOM is rendered, so you have to use event delegation to target its focus and blur events
$("body").on("focus", "input.combo-text", function(){
$(this).data("prev-value", this.value);
this.value = "";
})
.on("blur", "input.combo-text", function(){
if ($(this).val() === ""){
$(this).val($(this).data("prev-value"));
}
$(this).data("prev-value", "");
});
As you can see, when the input receives focus, we are storing the previous value as a data attribute on the element, then retrieving it when focus is lost if the user didn't input anything different.