How do I change the environment map reflectivity value in script

Hi, how do I change the environment map > reflectivity value in code?

I thought it might be:
meshInstance.material.my_cubemap.reflectivity = 1;

…but that just errors with undefined setting ‘reflectivity’

is using the following correct?
meshInstance.material.reflectivity = 1;

Note: I am manually assigning a cubemap to a material at runtime and it seems like its reflectivity is getting set to 0 during this process. That sound strange as the default setting should be 1, but I want to rule it out by setting the reflectivity manually.

Thanks