Oopps got it!
/// <summary>
/// Called when the node enters the scene tree for the first time.
/// </summary>
public override void _Ready()
{
MeshInstance mMesh = this;
Material mMat = mMesh.GetSurfaceMaterial(0);
ShaderMaterial mShaderMat = mMat as ShaderMaterial;
//--> Get the Plane parameter from shader
//
Plane mDefaultPlane = (Plane)mShaderMat.GetShaderParam("plane");
//--> Set new value on Plane parameter to shader
//
Plane mReplacePlane = new Plane(1, 1, 1, 0);
//
mShaderMat.SetShaderParam("plane", mReplacePlane );
}
