How to write left outer join for this lambda c# method

Question

I wrote the inner join for this C# lambda expression but i cant figure out the code for left outer join, here is the code for the inner join...

public List<Server> GetServerResponse()
{
  try{
       return _serverRepository.FilterBy(x=>x.IsActive && x.IsDashboardVisible).Join(_ServerHelperRepository.FilterBy(x=>x.type == Constants.ServerType), a => a.ServerID, b => b.ServerID, (a,c)=> new Server
         {
            OpeningCode = a.OpeningCode,
            ClosingCode = a.ClosingCode,
            OpeningThirdPartyCode = a.OpeningThirdPartyCode,
            ClosingThirdPartyCode = a.ClosingThirdPartyCode,
            AuditHistory = _auditTrailRepository.FilterBy(x=>x.ElementType == Constants.OutageType && x.ElementId == o.RequestId).OrderBy(z=>z.Id).ToList()
            
         }).OrderBy(Server=>Server.Name).ToList();
     }
  catch{
         (Exception ex) { return new List<Server>();}
       }
} 

This works perfectly but i cant seem to write it as left outer join