@Maniganda and for anyone who has this issue in future (as I know it's late for you now)

this.$el.find() is a method that is used to parse through the html of the current widget and find elements inside it based on their html class, id or element name, remember the scope of the search is local to only that widget, not to the entire document.

You can use it as following:

this.$el.find('#id_of_element')

this.$el.find('.class_of_element')

this.$el.find('name_of_element')