You can try to do this:
https://forum.proxmox.com/threads/invalid-csrf-token-on-every-edit.68209/#post-493746 (reboot server and clear cache, browser history ect and try again)
Here is more detail for
https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#pmgconfig_whitelist_overview
By the way that field is for user whitelist if you are trying to a global exception is better to add to Who Objects
https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#pmg_mailfilter_who
There are three different types of whitelist you can do.
First is
https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#_mail_proxy_configuration (Configuration - Mail Proxy - Whitelist - only add IP or network here or it won't work and restart postfix service for each exception to apply changes) This is for SPF, Unknown senders, DNSBL exceptions)
Second is
https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#pmg_mailfilter_who (you can add anything here it will bypass SpamAssasion filtering depending on whitelist priority accept action is final so no rules will execute after that)
Third is
https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#pmgconfig_whitelist_overview (4.4.3. User White-/Blacklist) - The user white- and blacklist are user specific. Every user can add mail addresses to their white- and blacklist.