First, check if your apache2 has the utils package
sudo apt-get install apache2-utils
Then, set the username and password.
sudo htpasswd -c /etc/apache2/.htpasswd <username>
After that, edit your reverse proxy to use the authentication
<VirtualHost *:80>
ProxyPreserveHost On
ProxyPass / http://someaddress:1234/
ProxyPassReverse / http://someaddress:1234/
Timeout 5400
ProxyTimeout 5400
ServerName dev.mydomain.com
ServerAlias *.dev.mydomain.com
<Proxy *>
Order deny,allow
Allow from all
Authtype Basic
Authname "Password Required"
AuthUserFile /etc/apache2/.htpasswd
Require valid-user
</Proxy>
</virtualhost>
At least, update your apache
sudo service apache2 reload