First, you have to create a separate section. Go to Settings -> Single Property Page -> Property Settings Tabs and provide section name there.
Then, you have to create a field inside that section. For this purpose, please go to Property Fields menu and drag a Shortcode field, set appropriate title and data name (title will not appear on the frontend) and choose the newly created section from Admin Setting Tab. Now save the changes.
Now, when you will edit a property, you will see a new text field under a new section. You can provide the iframe or other code from any 3rd party 360 tour service to that field and it will render it on the frontend.