Powershell script for Exchange 365 Mailflow Rule - filter for recipient who is ONLY a BCC

I understand that the website rules are unable to configure to match inbound emails where the recipient is ONLY a BCC, and we need to filter to just those BCC instances, and then Add to either the Subject or the pre-pend to the message body both a fixed "BCC" and the relevant variable but unsure which:
%%bccaddress%% or %%bcc%% or %%recipientemailaddress%%. or something else!

I understand that the Mail flow rules have more options via powershell scripts, but we have little expertise, so need a little help and some pointers please!

We want the recipient to KNOW they were BCC'd and what email address it came to them via (as we have several mailboxes, but also get spam (a similar domain name but different company we get lots of their emails - where they are BCC'd) and need to deal with it.

The next step will be to block those email addresses or set autoresponders/forwarders but not sure yet what we'll do, first things first to identify the BCC'd emails and what email they arrived via!
Thanks
Richard