Here is the documentation for the midi settings. Writing this in the the midi field:
my_midi:0,1
will create a midi connection to input 0 and to output 1, which widgets can send messages to (and receive from) if their target property is set to "midi:my_midi" ("my_midi" is just an arbitrary name of your choice). This
my_midi:UMC404,UMC404
would work too (« If a string is specified, the first port whose name contains the string will be used »).