Hi Anderson,
Given the date you posted this question, you might have already figured out the solution. But I hope it will be helpful to other users.
For the cursor to focus on the 'username' field on the login page, 'loginpageautofocus' option should be enabled.
To locate the page to modify this option, you can either search 'loginpageautofocus' in the global administration search field, or you can navigate to:
Site administration >> Plugins >> Authentication >> Manage authentication
and scroll down to "Autofocus login page form" section. By default, it is set to "No".
Thank you,
Gehendra
