Extra charges for b.com you have to do it directly in b.com extranet.
b.com does not have possibility to add extra charges like parking, cleaning fee, registration fee in myRent.
b.com only allows to set up promotions/discounts in myRent.
To add promotions/discount you can find it under this link:
https://myrenthelp.freshdesk.com/a/solutions/articles/44002141339