Redmine

LDAP now appears to be working properly with the following:
Authentication mode (LDAP)
Name *: DOMAIN_Redmine
Host *: server.co.com
Port *: 3268, LDAPS
Account: DOMAIN\ProcessRedmine
Password: xxxxx
Base DN *: DC=co,DC=com
On-the-fly user creation: X
Attributes
Login *: sAMAccountName
First name: givenName
Last name: sn
Email: mail

However, when a new user registers their authentication mechanism is set to internal, by default. I saw a not the mentions an LDAP preference somewhere, but I just can't find it:(... Can you explain exactly how to set the default authentication to LDAP? If this is not available in the GUI, can you please point me to the configuration file and the specific tweak required to accomplish this?