on AOT datasets fields method, add this code below :

void dataSetLookup(SysDataSetLookup sysDataSetLookup)
{
    List list = new List(Types::String);
    Query query = new Query();

    // Add the table to the query.
    query.addDataSource(tableNum(HcmLeaveType));

    //filter query
    query.dataSourceTable(tableNum(HcmLeaveType)).addRange(fieldNum(HcmLeaveType,ValidToCredit)).value(SysQuery::value(NoYes::Yes));

    /* uncomment if the relation is not based on reference recid
    // Specify the fields to use for the lookup.
    list.addEnd(fieldStr(HcmLeaveType,LeaveTypeId));
    list.addEnd(fieldStr(HcmLeaveType,Description));

    // Supply the set of lookup fields.
    sysDataSetLookup.parmLookupFields(list);

    // Specify the field that is returned from the lookup.
    sysDataSetLookup.parmSelectField(fieldStr(HcmLeaveType,LeaveTypeId));
    sysDataSetLookup.parmHideSelectField(true);
    */ 

    // Pass the query to the SysDataSetLookup so that the query is used.
    sysDataSetLookup.parmQuery(query);
}

preview after lookup filtered by code :

reference on MSDN :
https://msdn.microsoft.com/en-us/library/hh830903.aspx