Think that's called 'single sign on'.
Please see: https://moodle.org/plugins/browse.php?list=category&id=21
And a word of caution ... make sure whatever you setup does NOT change the manual auth method for your initial admin user in Moodle. If you boo-boo there you could end up where no one could login to the moodle to admin the moodle.
'SoS', Ken