If your web page is requesting a password, it is best to send the password over an encrypted channel.

Before your authentication block, add this code to redirect http://URL to https://URL:

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^/?(.*) https://%{HTTP_HOST}%{REQUEST_URI} [R,L]

Surrounding your authentication block, add this condition which forces authentication to only occur when your page is served over https:

<If "%{HTTPS} == 'on'">
  <Your authentication block>
</If>

That's all you need to do! Putting it together, here's an example:

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^/?(.*) https://%{HTTP_HOST}%{REQUEST_URI} [R,L]

<If "%{HTTPS} == 'on'">
  AuthType Basic
  AuthName "My Page"
  AuthBasicProvider pam
  require valid-user
</If>