eg. for domain www.mape.org.uk
The status access URL will be http://www.mape.org.uk/awstats/awstats.pl
The username will be ftpma004 (their standard FTP access username)
You will need to change their stats password to match their FTP password, via the following command (executed on the server hosting their website):
htpasswd -m /etc/awstats/awstats.pwd ftpma004
You will then be prompted to enter a password for the above user.
Each individual site with awstats has an individual config file under /etc/awstats and within each file their is a config entry to show what usernames should be allowed access.