//use the following command:
myDropzone.hiddenFileInput.click()

//to find dropzone instant there are several ways:
//1- by jquery: 
var myDropZone =  $('.dropzone').get(0).dropzone;
//or
var myDropZone = $("div#dropmehere").dropzone({...});  //if you are using id to find your element

//2- by Dropzone class itself: 
var myDropzone = Dropzone.forElement("div#dropmehere");