This was a pain in my ass and I was searching everywhere on theKnot and reddit, etc.
This is what you do.
On the top of theKnot website, you'll see (Planning Tools, Vendors, Wedding Websites) etc. You're going to click on wedding website, scroll down to YOUR GUESTS & RSVPs, and click on online RSVP. From there, click on ADD QUESTION, and you'll see an example of WHAT IS YOUR MEAL PREFERENCE. List your dinner here. When the guest clicks on it, you should have a few options for them to select. Mine was steak, fish, or vegetarian. You get my point. I hope this helped you guys out.