This ain't easy, nor will always work fine as A and D strings note frequencies can also be played fretting E and A strings…. so it would trigger the fx when you don't need it.
Anyway there is a PITCHDETECTOR module which has a frequency output. This is useful for frequency based "conscious" applications. Then a COMPARATOR/COMPARATOR2/C_COMPARATOR module may be used, along with some other math and control modules to realize what you need.
best
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