Using the Plesk Control Panel to password protect directories:
Assuming your /phpmyadmin directory is at:
/home/httpd/vhosts/domain.com/httpdocs/phpmyadmin
Then go into the Domain module
Click on the Protected Directories icon
Click on Add new directory
Enter the path /phpmyadmin
Select Non-SSL or SSL or both
Fill in optional Header text.
Click on Ok
Second part - create user/passwords
Click on the link /phpmyadmin
Click on Add new user
Fill in username
Fill in password
Fill in confirm password
Click Ok.
Repeat the second part for any additional user/pwds you desire.
Plesk does not store this info in .htaccess or .htpasswd files. Looks like they keep the auth file at:
AuthUserFile /home/httpd/vhosts/domain.com/pd/d..phpma
and ~/domain.com/conf/httpd.include:
<Directory "/home/httpd/vhosts/domain.com/httpsdocs/phpmyadmin">
AuthType Basic
AuthName "Login Required for phpMyAdmin"
AuthUserFile /home/httpd/vhosts/domain.com/pd/d..phpmyadmin
require valid-user
</Directory>
Note: Any changes you manually put into httpd.conf or httpd.include may be overwritten by Plesk without warning.
Better to use vhost.conf, vhost_ssl.conf, .htaccess for customizations.
If you prefer the .htaccess/.htpasswd method, then I would suggest that you switch to using SSH instead of Telnet. Once you login using SSH, it's just like connecting by Telnet, but secure! Then you can create your .ht* files as you used to.
See httpd://www.ssh.com