Taken from the Warp 7 manual that can be downloaded from the H&K website:

FOOTSWITCH: Jack designed to take a standard
footswitch (the HUGHES & KETTNER® FS-1 will do
nicely) for switching back and forth between the
CLEAN and WARP channels. To do this, the
CHANNEL SELECT switch on the front panel must
be set to CLEAN (red LED does not light up).

Looks like at least the first switch you linked to should work.