Sorry, we don't know exactly what a "Boss fs-5l" is... The recommended pedals for your Yamaha MX are as follows: Yamaha makes an FC3 (sustain pedal with half-dampering), FC4, or FC5 as sustain type "momentary" pedals and the FC7 as the sweep type Foot Controller... please see your Owner's Manual, page 22. The MX has a SUSTAIN jack and a FOOT CONTROLLER jack. Any pedal you plug into the SUSTAIN jack will default to controlling sustain (hold).

I'm sure it (the Boss) is just a simple latching/unlatching switch... all we can suggest is you try it... you don't mention what you want this pedal to do (besides "make it work in all my patches and performances") work? to do what? I would imagine you would plug it into the sustain jack... My guess is that this will not be satisfactory as it would not work like a sustain pedal... you click it and things would sustain... you'd have to click it again to unsustain(sic)... you cannot hurt anything, but I cannot imagine that would be a satisfactory way to work sustain. Is that what you are attempting? You named the thread "Hold Function"... "HOLD 1" in MIDI is Sustain.

We recommend the Yamaha FC3 sustain pedal for the MX.