If you have another DN for your Anonymous Username, you have to check Search First and give all ldap "characteristics" for it.
Example :
LDAP Hostname : yourLdapHostname.com
Base Distinguished Name : OU=UsersEmea,DC=emea,DC=msad,DC=mycorp
Search Attribute : sAMAccountName
Check Search First
Anonymous Username : CN=myanonymous,OU=otherservice,DC=emea,DC=msad,DC=mycorp
Anonymous Password : <password of myanonymous>