Poweshell Command for Get-Mailbox

I am trying to get the mailbox using powershell cmdlet in c# program. I can see connection is being initialized successfully but when try to execute the get-mailbox cmdlet, i see the following error.

The term 'Get-Mailbox' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
   at System.Management.Automation.Runspaces.PipelineBase.Invoke(IEnumerable input)
   at System.Management.Automation.Runspaces.Pipeline.Invoke()

below is my c# code

Command cmd= new Command("Get-Mailbox");

                    cmd.Parameters.Add("Identity", "xxxxxx");

                   

                    Collection<PSObject> results = new Collection<PSObject>();

            runspace = RunspaceFactory.CreateRunspace();
                    runspace.Open();


                    Pipeline pipe = runspace.CreatePipeline();

                    

                    pipe.Commands.Add(Cmd);

                    

                    results = pipe.Invoke();