To instantiate a creation popup, it is necessary to provide it with an input component (here newTask):

var newTask = new Task();
var popup = JSAPI.get().getPopupAPI().buildComponentCreation(newTask);

A callback can be provided to this method to react to the creation of the component:

var newTask = new Task();
newTask.setClassId("ClaimProcess_Start");
var popup = JSAPI.get().getPopupAPI().buildComponentCreation(newTask, function(saved){
    console.info("The task has been created: "+saved.getId());
});
popup.show();

File selector

In the case of document creation, it is also possible to display a popup without a file selector. This instantiates a blank document with no content.

To do this, use the buildDocumentCreation() document-specific function. This function has a parameter allowFileAttachments, which allows the file selector to be displayed or not within the popup.

var newDocument = new Document();
var popup = JSAPI.get().getPopupAPI().buildDocumentCreation(newDocument, false);