To do it with scripting you'll need to use the setAttribute function to assign a value to the waveform attribute of the waveform generator. At the moment all you're doing in your combobox callback is calling the getId function which doesn't do anything. Also you're passing a parameter to the function which will produce an error because that function doesn't take any parameters - check the docs.
@d-healey Beware, it's mad sketchy. I played around with showing the impulse response on my front end for months and got tons of crashes. Whenever you move a knob related to the convolution, the waveform will flicker in a really annoying way and often crash the plugin. I think I posted about this elsewhere with the crash report.