@Mathieu347 It's not against Airbnb policy to include a private chef as an amenity, but they don't offer this as a search filter for the small portion of guests who might happen to be seeking it out. As a result, there's no competitive advantage to be gained by including it in your nightly rate.
You can offer many kinds of optional services as add-on items, including prepared meals, using the Resolution Center. The smoothest way to do that is to invite guests to choose specific services at specific times (e.g. one dinner on Friday night, one daily breakfast, or one interim cleaning per week) and use the Resolutions tool to collect pre-payment before delegating the labor. This is not the 19th century, and Airbnb guests aren't generally looking for in-home servants to be in the house with them, so it should be very clear that what you are offering is the service, rather than the human being who is performing it.