I have always had to use
mydomain\userid to log into DRA.
The base search path I use for DRA is similar to
dc=mydomain,dc=ad,dc=company,dc=com
For DAM I can log in with just a
userid, and there are two paths in the external system --
Users OU=dept,OU=division-users,OU=all-users,dc=mydomain,dc=ad,dc=company,dc=com
Groups OU=groups,dc=mydomain,dc=ad,dc=company,dc=com
I tested the DRA base search using search strings similar to DAM, and neither search string worked.