Adding a virtual tour to Airbnb isn't straightforward. They don't have a field you can just fill out with a link to your tour. You can still add it through this roundabout way though:

Steps

Build & publish your tour.
(Optional) Shorten your tour URL by routing it through bitly or TinyURL
Paste your URL into your listing's description (ex. "Visit this link for virtual tour: bit.ly/virtualtour" -- Please note that Airbnb doesn't allow hyperlinks in descriptions, so bookers will have to manually type in the URL in their browser to visit it, which makes a shortened URL even more important).
(Optional) Generate a QR code from your URL and add it as one of your listing's images, so mobile users can more easily visit it.