As with every provider, the Parallels provider has a custom box format.
The actual list of official boxes provided by Parallels is available on wiki page.
All boxes for "parallels" provider could be found on Vagrant Cloud: https://app.vagrantup.com/boxes/search?provider=parallels
Adding a box from the catalog is very easy:
$ vagrant box add bento/ubuntu-18.04 ...
You can also quickly initialize a Vagrant environment with the command:
$ vagrant init bento/ubuntu-18.04 ...