How to add an attribute to an LDAP schema - active-directory

Question

I am trying to add a new attribute named sAMAccountName to an already existing LDAP schema definition which is read by IM-LDAP using UnboundID LDAP SDK.
I have added an attributeTypes entry and sAMAccountName to matchingRuleUse.
attributeTypes: ( 2.5.18.11 NAME 'sAMAccountName' DESC 'MS Sec Principal User' EQUALITY distinguishedNameMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.12 SINGLE-VALUE NO-USER-MODIFICATION USAGE directoryOperation )
matchingRuleUse: ( 2.5.13.1 NAME 'distinguishedNameMatch' APPLIES ( creatorsName $ ... $ sAMAccountName ) )
For every previously existing attribute a call to com.unboundid.ldap.sdk.schema.Schema.getAttributeType("attrName") returns the attribute type. But not for my new attribute.
What am I missing?
Edited 10/11/18 after #jwilleke comment:
What I am trying to achieve is a mock using IM-LDAP for a very limited functionality of an Active Directory server.
In the actual AD production environment, there are entries representing users with objectClass: person, organizationalPerson,simulatedMicrosoftSecurityPrincipal.
In AD these entries contain sAMAccountName and memberof attributes.
But they are not there in the schema that comes with IM-LDAP.
The authentication Java code first performs a search on
(&(objectClass=user)(sAMAccountName=userAccountName)
Then if an entry in found, it checks whether a given security group name is present in the multivalued attribute memberof.
An entry exported from the production AD server looks like this :
dn: cn=Smith\,John,ou=User Accounts,dc=ACME,dc=CORE,dc=INT
changetype: add
objectClass: person
objectClass: organizationalPerson
objectClass: simulatedMicrosoftSecurityPrincipal
cn: Smith,John
sn: JohnS
sAMAccountName: JohnS
userPassword: johnspasswd
memberof: ou=Service Accounts,dc=ACME,dc=CORE,dc=INT
The two new attributes are added to objectClass simulatedMicrosoftSecurityPrincipal in the schema by adding:
objectClasses: ( 2.5.6.24 NAME 'simulatedMicrosoftSecurityPrincipal' DESC 'MSSecurityPrincipal' SUP top AUXILIARY MUST sAMAccountName MAY memberof )
But when I try to import these entries into the LDAP mock I get an error telling me that sAMAccountName and memberof are not defined.
EDIT 2:
attributeTypes: ( 1.2.840.113556.1.4.221 NAME 'sAMAccountName' SYNTAX '1.3.6.1.4.1.1466.115.121.1.15' SINGLE-VALUE )
Wokrked for adding sAMAccountName, but still trying to figure out how to add memeberOf
Here is the complete .ldif file.

Answer 1

This is what finally worked for me
objectClasses: ( 1.2.840.113556.1.5.6 NAME 'microsoftSecurityPrincipal' DESC 'MS SecurityPrincipal' SUP top AUXILIARY MUST ( sAMAccountName $ memberOf ) )
attributeTypes: ( 1.2.840.113556.1.4.221 NAME 'sAMAccountName' SYNTAX '1.3.6.1.4.1.1466.115.121.1.15' SINGLE-VALUE )
attributeTypes: ( 1.2.840.113556.1.2.102 NAME 'memberOf' SYNTAX 1.3.6.1.4.1.1466.115.121.1.12 )