Save as /etc/yum.repos.d/proxmox-stable.repo (channel stable).

[proxmox-stable]
name=Proxmox stable
baseurl=https://repos.onesystems.ch/rpm/proxmox/stable
enabled=1
gpgcheck=0
repo_gpgcheck=1
gpgkey=https://repos.onesystems.ch/api/projects/proxmox/gpg-key

After saving the file, run dnf update. DNF imports the GPG key from gpgkey= automatically — confirm the prompt once.