You need to set an IP or IP range to have access.
http-access 0/0 admin means all IPs have admin access.
There is not a pmta configuration option to set a username and password.
If you want to protect pmta with a username and password, or some other authentication, you'll need to put some other software like a proxy in front of it. apache, haproxy, and many others could be configured to do what you need.
