Hi Hai,
My first look is your login page is different with the video, the video 0:00 shows a Nanoflow that create an "Anonymous" object to be returned. Mean that "Anonymous" object somehow is passed or as the data source in the login page as page parameter.
In login page (0:06), the page parameter is access through "data view" widget, there is a container with blue color header written Anonymous by nanoflow 'DS_Anonymous_Nanoflow'. Hence the button redirects to register page with "Anonymous" object as page parameter will take the object from the login page in the "dataview".
In the video case, this documentation might help you to have a general knowledge about the dataview, which provide any widget within accessing to data you selected: