thanks, didnt know how to do that

1. open install script, in this case it was https://downloads.nordcdn.com/apps/linux/install.sh
2. look for repo url
BASE_URL=https://repo.nordvpn.com/
KEY_PATH=/gpg/nordvpn_public.asc
REPO_PATH_DEB=/deb/nordvpn/debian
REPO_PATH_RPM=/yum/nordvpn/centos
RELEASE="stable main"
ASSUME_YES=false
3. look for fedora/rpm/dnf/yum
REPO_URL_RPM=${BASE_URL}${REPO_PATH_RPM}
install_dnf() {
    if check_cmd dnf; then
        get_install_opts_for_dnf
        install_opts="$RETVAL"
        
        repo="${REPO_URL_RPM}"
        if [ ! -f "${REPO_URL_RPM}" ]; then
            repo="${repo}/${ARCH}"
        fi

        $SUDO rpm -v --import "${PUB_KEY}"
        $SUDO dnf config-manager --add-repo "${repo}"
        $SUDO dnf $install_opts install nordvpn
        exit
    fi
}
99% of the time you probably use x86_64 arch
so their repo is https://repo.nordvpn.com/ + /yum/nordvpn/centos + x86_64
4. create repo file
[name]
name=Pretty Name
baseurl=url that you extracted
enabled=1 -> enable repo
5. if they provide key like package.asc then find the url
PUB_KEY=${BASE_URL}${KEY_PATH}
so the url is https://repo.nordvpn.com/ + /gpg/nordvpn_public.asc
then add this line to repo file
gpgcheck=1 -> check the gpg
gpgkey=url that you extracted