We don't use tableau online so I may be talking out of my hat, but our embedded dashboards have a pop up screen that asks users for their login credentials before it allows them to see the viz.
I'd envisage that your licenses would probably work in the same manner.
In terms of access control, this is done via the web interface screen, where you set users and groups, so that different sets of users would have access to different visualisations.
Thank you for your response, In Tableau Server, I used to generate a token for each user who login to the web application and without any pop-up/login request we allowed user to access the visualization.
But Online is restricted with user and trying to see how to make them to access the visualization with just web application login(making second login is user inconvenience, feel).