Hello @Julia931,
There isn't a way to add a pet fee so that's automatically included in the total charged to the guest.
All you can do is to prominently mention it in your listing and in messages to the guest.
You then have to use the resolution centre to request the money from the guest. Once the guest accepts the request the charge is taken and paid to you in the usual way. If they don't accept it no charge is made.
To request money, go to the reservation in question and click the "Send or request money" item in the options and follow the procedure from there.
Steve.