[_serial] Tag – how to use?

Flamingo 1.5 installed
I use Contact Form 7 and put [_serial] into my form content. The sent mail shows me an incrementing number. I also tried putting flamingo_number: “[_serial_number]” into “Additional settings” of CF7 form, but it get’s ignored by flamingo. Mails get stored in flamingo, the Inbound view shows me a column called “Tags” with “No tags” as content for every inbound mail.

Question: How do I store a serial number of a CF7 message to flamingo as well? Adding an ID to a sent mail is useless, when you can’t use that ID to easily find and reference a stored version with it.