How to programatically set ShaderGraphMaterial properties?

I have successfully loaded a ShaderGraphMaterial object from a Reality Composer Pro usda file -- here is a the graph program:

When I attempt to set a parameter

try shotTrailMaterial?.setParameter(name: "TimeDelay.Value", value: .float(1.0))

I get a parameterNameNotFound exception. What am I missing? What is the trick to defining and/or accessing these shader parameters?

Here's a sequence of actions to create MaterialX controlled parameter in ShaderGraph:

• select YourMaterial in RCP left pane
• in Inspector, choose New Input
• add Name and correct Type of parameter
• click Add (this adds a blue node to the Graph)
• connect this node to any node's input you're going to control
• then in RealityKit, use the following code