To use an external switch for tap tempo, you have to assign that function to that switch so the H90 to recognize it and know what to do.  It may be that your current switch will work but you have not assigned it properly in the H90.  I suggest you first make sure you have assigned that switch properly before making a new one.   Follow the manual to do that.  If you are unsure which aux switch will work, plug it in and try all of them until you get it.  Restated, assign aux switch 1 and test.  No worky? Assign aux switch 2 and test.  Etc up to aux switch 6 until you get it.

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If that does not work, you can make a one button switch/box.  That is VERY easy to do.

You need a normally open momentary switch.  Single pull single throw (spst).

A TS cord (two conductor – a signal (tip) conductor and a ground (sleeve) conductor – that’s a regular guitar cord) with a 1/4 inch plug.

If you are going to hardwire the cord to the switch in the box, solder the signal conductor to one of the two terminals of the switch.  Then solder the ground conductor to the other terminal of the switch.  There are only two terminals.  There are only two wires.   That’s it.  No diodes needed for this one button switch.

If you want a 1/4 jack on your footswitch box, you obviously follow the same wiring steps to add that rather than hardwiring the cord directly to the switch.

Now plug that into the back of the H90 and follow the manual to program/assign that switch to perform tap tempo.   As discussed above, if you are unsure which aux switch will work try all of them until you get it.

Good luck! You can do this!