Your code is syntactically incorrect.
var aa = db.LoanApplications.Where(b => b.ApplicationNumber.ToString() == ApplicationNumber).Select(b => b.ApplicantId.ToString()).ToList();
My best advice for fluent LINQ is:
1. Apply your logical filter (ie .Where())
2. Apply your select, if any.
3. Apply your ordering ( ie .OrderBy())
4. Enumerate (.Single(), .First(), .ToList(), etc)