@Madelyn1
Announce the extra fee clearly in your listing and also in your House Rules (they are what guests agree to when signing up).
As @Clare states, you can then charge your fee through the Resolution Center.
However, once you've charged something through your one-time option of doing this per booking, you wouldn't be able to use The Resolution Center again in case of damages or otherwise for said booking.
The other option you have is to charge by way of booking change. If guests don't accept it before arriving, you'll have to get them to accept before you let them into your listing. "Oh, I just noticed you haven't completed your reservation to include the dog. Let's just finalize this right now, and then you can get comfortable."
If you let them in before they've agreed to the change and wait in hopes of it happening the day after or the day after that, it'll probably never happen...