$expand and $ref

Hello,

I'm new at dynamics 365 API requested, being a dummies I have an issue when expanding a column.
I learn how to write, and it works:

"https://microsoftsales.api.crm.dynamics.com/api/data/v9.1/leads?$filter=Microsoft.Dynamics.CRM.OnOrAfter(PropertyName=@p1,PropertyValue=@p2)&@p1='createdon'&@p2='01/01/2019'&$expand=createdby($select=_siteid_value;$filter=_siteid_value eq xxxxxxxxx),&$select=_ownerid_value,
_createdby_value, estimatedamount_base, revenue_base, msp_cssdisposition, _campaignid_value, createdby, leadqualitycode, _accountid_value, numberofemployees,
_parentaccountid_value, msp_primaryproductcode, subject, msp_leadnumber, msp_estimatedquantity, msp_countrycode, numberofemployees, createdon"

However, it gives me all data, I only want the ones associated with the $filter=_siteid_value eq xxxxxx. I tried to use $ref but it didn't work.
Can someone help with this issue?

Many thanks in advance.