You can use cas.authn.pm.ldap[0].searchFilter , just make sure you set others required,
https://apereo.github.io/cas/6.5.x/password_management/Password-Management-LDAP.html , something like the below would work, msds-parentname is a constructed attribute in AD that holds the value of the current objects OU
cas.authn.pm.ldap[0].searchFilter=(&(objectClass=person)(!msds-parentdistname=OU=YourOU,DC=Example,DC=Com)(sAMAccountName={user}))
Jason