How do I blacklist on a custom header?

Given the following message Raw source:

X-Threatsim-Header: http://threatsim.com/speartraining?id=5770d1f44a
X-Ms-Exchange-Organization-Avstamp-Mailbox: MSFTFF;1;0;0 0 0
Content-Type: multipart/mixed; boundary="–==_mimepart_589b7f8d1a207_a6b2e456b87846572"; charset=“UTF-8”
X-Mailtags-Version: 4
X-Threatsim-Id: 5770d1f44a

How do I create a blacklist rule to key of of the specific header? If Header exists it is SPAM

For example -> X-Threatsim-Id or X-Threatsim-Header? The blacklist has limited headers to key from - its not “Return Path” or "Received (any)

The blocklist does not currently support rules with custom headers. However, you could make a rule in Mail (above the SpamSieve rule) that looks for this header and moves the messages to the Spam mailbox.