New PM custom arguments

Hi, I am adapting a pass plugin for opt to the new Pass Manager and
everything is fine but the arguments that this pass requires. The next
line works with the legacy PM and --xxx (or whatever) is understood:
  cl::opt<string> OutputFilename("xxx", cl::desc("This is a
parameter"), cl::value_desc("xxx"));
But this is not the case for the new PM. What can I do?
I have read an answer to a similar question that suggests to build the
pass inside LLVM, but there should be a better way to accept
arguments. Other than that, I can't find anything about this issue.