In TrueNAS, make a user with the Read-only Admin role, sign in as it, then open the top-right user menu > My API Keys > Add. Paste the key here. Its certificate is pinned, so the key only goes to your NAS.

In Proxmox: Datacenter > Permissions > API Tokens > Add (user root@pam, ID ultrobot, keep Privilege Separation on). Then Permissions > Add > API Token Permission: path /, role PVEAuditor (read-only). Paste it as root@pam!ultrobot=<secret>.

Home Assistant tokens can't be read-only, so make a separate non-admin user called ultrobot, sign in as it, then go to Profile > Security > Long-lived access tokens > Create token. Paste it here. You can revoke it any time.