How can I customize uniform variables that are used in replacement shader code in VTK.js?

In VTK, I can obtain a shader object from actor.GetShaderProperty() and customize uniform variables using the GetFragmentCustomUniforms().SetUniform3f() method. However, in VTK.js, the actor object does not have a shader property . How should I proceed to achieve this in VTK.js?

Have you tried something like this?