The masthead has two sheaves. One on the aft facing section where the backstay attaches and the other in the main part of the mast. You need to pass the messenger line over both. Tie some fishing line onto a section of bicycle chain about 8" long. Use a straightened coat hanger to push the chain twisted sideways over both sheaves, then rotate it so it bends and starts dropping inside the mast. The chain should start to pull the fishing line. You can fish the chain out the mast exit using a coat hanger bent to form a small hook. Then tie a stronger messenger line and pull that through. Once that is done, you should be able to attach the halyard to the messenger line and pull it through.
Put a reeving splice in the end of the halyard to make it easier.
ps - what hull number do you own?