OCaml Bindings: usage of the setGC function

Hello everyone...

I'm currently trying to use the [set_gc gc f] function without success. The
function type is "string option -> llvalue -> unit"

I have tried

set_gc "shadow-stack" func;

"Error: This expression has type string but an expression was expected of
type
string option"

and

set_gc ~gc:"shadow-stack" func;

"Error: The function applied to this argument has type Llvm.llvalue -> unit
This argument cannot be applied with label ~gc"

Is anyone aware of the correct syntax for using this function?

Thanks,
Frank