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
and add your provider like this
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
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