How do I post a result for a workout that isn't on the site?

You'll need to create the workout first. Click "find"=>"workouts" and on the top of that page you can click the link "Create one". Click that and you'll go to the workout creator where you can set up a new workout, pick what posting type, how many rounds, etc. If you try to make a workout that already exists you should get a warning that says the workout already exists, with a link to that workout so you can post to it.