Portamento with a Foot Switch has been a MIDI reality since 1983.
Portamento On/Off = cc65
If you assign the FS = 65, it will act as the Portamento Part Switch in each KBD CTRL Part.

Press [UTILITY] > “Settings” > “MIDI I/O” > FS ASSIGN = 65

What is it you are attempting to do?
Each Performance has a Master Portamento Switch
Each Part has a Portamento Part Switch and settings for its behavior.
CC65 operates the Part Switch for each Part with Portamento settings

Should also be able to use a MIDI foot switch which allows for targeting specific MIDI channels for those channel CCs. Assuming keyboard is in multi-channel MIDI mode, not sure one way or the other at the moment if Portamento decodes by channel or not. Regardless, MIDI foot controllers typically allow for more flexibility and allow the foot switch to remain open for assigning some items which can only be done using the footswitch (like live set advance).

Not saying to purchase a MIDI foot controller - this may not fit your budget or complexity threshold for what you want to do. Just adding to the list of available options.

There is no reason to purchase a MIDI Foot Switch as it provides no advantage in this instance, since controlling Portamento On/Off can be on a per Part basis... it is typically not necessary to turn the Part Portamento Switch on a Channel you are not actively playing. And each MODX Part has its own Portamento Switch... which determines if Portamento is active or not regardless of MIDI I/O Mode (Multi or Single).

While dedicating your Foot Switch to cc65 is a global assignment, you have options to send cc65 from controllers other than the Foot Switch.
For example, the [MS HOLD] and [MS TRIGGER] buttons can be programmed as a latching Switch, or a momentary Switch, respectively.
An FC7 can be set to send cc65 which turns it into a Portamento ‘switch’, or setting the Ribbon to cc65. All of these are programmable on a per Performance basis.

From HOME
Press [EDIT]
Touch “Control” > “Control Number”
Here you can decide what CC messages are sent from this Performance

...Which is why I asked, what are you trying to do? If it’s a matter of ‘hands being busy’, get yourself a second Yamaha FC7 or reprogram the one you have for this particular Performance. Please don’t spend time looking for a MIDI Portamento Switch... you’ll wind up spending far too much especially if this is all you need it to do. An additional FC7 will serve you better at fraction of the price. And given the number of built-in options... there is simply no need.

It's good a more complete list of possibilities is being documented to allow the user decide what fits best.