I am not familiar with webmin, so not sure what kind of interface you have if any. I assume you know how to do the time part and are just looking for the actual command. So this is what I use in cpanel, hopefully it's the same.
There are several options depending on your particular server and the path to php. Try each until you get one that works. Set it for one minute so you can check results.
This is the one that works for me
php -q /home/username/public_html/autopost/update-feeds.php
others are:
/usr/local/bin/php /home/www/website.com/autopost/update-feeds.php
GET
http://www.website.com/autopost/update-feeds.php > /dev/null
php www/website.com/autopost/update-feeds.php
cd /home/www/website.com/post ; /usr/local/bin/php /home/www/website.com/update-feeds.php
or as a last resort you can always use a desktop cron tool like EzCron