Try creating scriptfile as a separate command:

Command myCommand = new Command(scriptfile);

then you can add parameters with

CommandParameter testParam = new CommandParameter("key","value");
myCommand.Parameters.Add(testParam);

and finally

pipeline.Commands.Add(myCommand);

Here is the complete, edited code:

RunspaceConfiguration runspaceConfiguration = RunspaceConfiguration.Create();

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

RunspaceInvoke scriptInvoker = new RunspaceInvoke(runspace);

Pipeline pipeline = runspace.CreatePipeline();

//Here's how you add a new script with arguments
Command myCommand = new Command(scriptfile);
CommandParameter testParam = new CommandParameter("key","value");
myCommand.Parameters.Add(testParam);

pipeline.Commands.Add(myCommand);

// Execute PowerShell script
results = pipeline.Invoke();