This is not the correct syntax.
ldapsearch -x -b "DC=mydomain,DC=local" -H ldap://192.168.1.1 -D "CN=Administrator,CN=Users,DC=mydomain,DC=local" -W "objectclass=user" -W sAMAccountname
remove the trailing "-W sAMAccountname
The final query would be:
ldapsearch -x -b "DC=mydomain,DC=local" -H ldap://192.168.1.1 -D "CN=Administrator,CN=Users,DC=mydomain,DC=local" -W "objectclass=user"