We need to find right url at first.

open this page https://app.vagrantup.com/boxes/search

choose you image

choose your version

copy the link

https://app.vagrantup.com/generic/boxes/centos8/versions/2.0.6

and add your provider like this

https://app.vagrantup.com/generic/boxes/centos8/versions/2.0.6/providers/hyperv.box

and now you have full url for vagrant box

put the url into browser and downlod the box and saved as like “centos8.box” in folder where you have installed vagrant.
In my case

~/.vagrant.d/boxes/centos8.box

add box in vagrant with following command:

vagrant box add generic/my_centos8  ~/.vagrant.d/boxes/centos8.box

check if vagrant box exists

vagrant box list