Maya USD define Usd Primitives on the stage using Python for MayaUsdProxyShape

maya_usd_define_primitive.py

# Tested December 21st, 2020
# This is just a test example of how one could use the USD Api against a mayaUsdProxyShape and its USD stage.
# Note: The outliner doesn't really like it when you change the stage
#       so you might need to toggle shape display off and on to get it to update.
import mayaUsd
from maya import cmds
from pxr import Usd, UsdGeom

shape = cmds.ls(type="mayaUsdProxyShape", long=True)[0]  # make sure to use long full paths 
stage = mayaUsd.ufe.getStage('|world' + shape)
obj = UsdGeom.Xform.Define(stage, '/test_group')

The type name start with lower case letter, to be mayaUsdProxyShape

The type name start with lower case letter, to be mayaUsdProxyShape

Updated. Not sure why it worked for me at the time.