The classes in the API that might be of interest to you are DspNetwork and Node.
Now, to change the actual value of a parameter you need this hidden class
Link Preview Image HISE/hi_scripting/scripting/scriptnode/api/NodeBase.h at 03c420c1d12f7a4457a8b497a6b78bc49d250e85 · christophhart/HISEThe open source framework for sample based instruments - HISE/hi_scripting/scripting/scriptnode/api/NodeBase.h at 03c420c1d12f7a4457a8b497a6b78bc49d250e85 · christophhart/HISE
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So give it a try. First you'll have to find out the name of the dropdown parameter, then you could maybe try to pass ints 1, 2, 3... or 0, 1, 2... or a straight up string.