PxrMatteID outputs matte AOVs for compositing.  This requires you add a User Attribute, explained in more detail here.

Add a user attribute for MatteID0.

Set its type and color value.

Add PxrMatteID

In your PxrMaterialBuilder Network, add a PxrMatteID VOP.  Set the color as the user attribute above.

Connect PxrMatteID's resultAOV to PxrSurface's Utility Pattern.

Set up AOV for Output