I always ask immediately after making the reservation by messaging the host explaining that I have an EV and saying each host has a different policy and what would be their policy if I were to hook up for a few hours overnight and of course I would be prepared to pay. The chances are that they have never considered it and when they do there is one of three reactions: 1, that will be fine you could plug it in at... 2, How much would it use? (chances are they've no idea and think it will cause a local blackout or fry their house wiring). 3, they have a policy which is either flat out no or certainly there's an outside socket you can use.
It boils down to being considerate and prepared to engage in a little education.. Earlier this summer we state at 2 AirBnBs and the first asked lots of questions and was clearly ill-informed. They asked how big the car battery was and How long it would take and questions about extension leads etc. I explained that compared to a dedicated EV Wall charger the adapter with a 13A plug was really only a slow trickle of a charge drawing less than a kettle but more than a toaster and roughly equivalent to the electric oven they had in the property. I explained that typically when staying at an AirBnB that we'd in the past just topped up 2 or 3 times during the week for a couple of hours in the evening if we'd not come across a suitable public charger in the course of travels and that having the facility even if we didn't use it would make our stay with them a much more pleasant and enjoyable experience. It got a very positive reaction and as it happened we came across an Instavolt charger and didn't need to use the facility. When we spoke to the host on leaving, she said her husband had been considering it and thought by next year they'd have an EV Charger point installed.
The following week, we stayed at a lodge where the owner had an external 3-pin socket and said we could charge there and let them know at the end of the week how much we'd used and leave money to cover it based on 17p/kWh which was great so we chartered to 80 overnight. Turned out that both her father and brother had a EVs.