How to make use of -Mm flag for multiple input ports?

See LISTSERV 16.5 - CSOUND Archives

This feature to enable access to multiple midi ports was added in 6.14, but I can’t figure out how to make use of it in ctrl7 (for example).

Say I want to use channel 3 of port 2, that would be channel 18 according to the docs, but I get an error when I plug that number into the ctrl7 opcode (which accepts 1-16). Any ideas what I’m missing?