Probably you mean the metadata arg, used with key ‘specs’. I meanwhile got the habit to almost always write SynthDefs with metadata specs.
With standard lib you can use ‘makeGui’ (not widely used this way it seems), with miSCellaneous installed ‘sVarGui’.
// using 'myFreq' here, as for 'freq' and 'amp' there are already global specs defined
(
SynthDef(\test, { |myFreq, amp|
Out.ar(0, SinOsc.ar(myFreq, 0, amp))
}, metadata: (
specs: (
myFreq: [20, 10000, \exp, 0, 400]
)
)
).add
)
SynthDescLib.global[\test].makeGui
\test.sVarGui.gui