Separate frontend access protection on the second page of my multisite

Hello, I'm looking for a way to put a separate frontend access protection on the second page of my multisite. Say there is domain.de, which is already live and there is a second page sudomain.domain.de running on a subdomain. I would like to protect the page on the subdomain with a frontend login. Like a htaccess protection.

I have already tried so many plugins and I really hope someone can help me now.