How to fill pop up form which has two fields 'username' and 'password'?

Question

I am learning Selenium, and got stuck at a case where at the starting of URL an alert pop-up generates and asks for username name password on the same alert pop-up. here is the picture?image of pop up with username and password How can I deal with it? there is no way I am able to select the fields with any of the methods and also when I print the page source nothing is printed. image of inspection of page showing no html content