Subsite admin url without the subdomain

Hi !

For my Wordpress Multisite SAAS, I would like to allow users to login through the main site "mysite.com" and manage their own site through a url like this "mysite.com/wp-admin". Right now, the url is "subdomain.mysite.com/wp-admin". My user can only create one site. I know it's possible because Happytables does that.

I also would like that when they logged out, they are logged out of their site AND the main site too.

Could you please help me ?

Thank you !