Open The button properties where you want to put this
command.

Click on RC5x and Enter in 16 64 13

That should be the code for THX.

When I purchased my SR9600 I went hunting for every code
available - I received the official document from Marantz
Japan which listed more than the original remote, and
on my own I found a few extras that even it didn't have.
However, there were no discrete THX codes - but really,
since the receiver should remember the audio processing
mode(s) on an input-by-input this shouldn't be too much
of an issue.

Are you saying that when you're in THX Mode on DVD and
you switch to Tuner and set it to Neo:6 Music and then
go back to DVD it stays on Neo:6? Because that's decidedly
NOT how my current SR9600 and old SR-18EX work - they'll
remember whatever mode you were in for each audio input,
and for the digital ones they'll remember several modes
(depending on the incoming audio source - ie a 2.0 mode
and a 5.1-only mode).

My TSU7000 setup contains the end results of my codes
search; the SR8500 just supports somewhat fewer than listed.

For the 9600 there are everal more codes pertaining to
THX
RC5x entry...
16 37 58 THX Surround EX
16 37 36 THX Cinema
16 64 07 THX Ultra 2
16 64 04 THX 5.1 Music
16 46 15 THX Games