How to add custom properties at the body level in SCDM?

# Python Script, API Version = V21
from SpaceClaim.Api.V21 import CustomProperty, SimplePropertyDisplay, Application
 
class CustomPropertyDisplay(SimplePropertyDisplay):
    """This is a property diplay class used to populate the properties window.
       To use it, you must first register it with the Application:
       Application.AddPropertyDisplay(...).
       Once registerd the GetValue method is called on each selection change, and it should
       return the string to be displayed."""
    
    def __init__(self, category, name):
        SimplePropertyDisplay.__init__(category,name)
    
    def GetValue(self, body):
        # Make sure the selection is a body
        if not isinstance(body, IDesignBody):
            return None
            
        # Try to get the "MyCustomProp" attribute from the body
        (success, value) = body.GetMaster().TryGetTextAttribute("MyCustomProp")
        return value
    
    def SetValue(self, body, value):
        # Make sure the selection is a body
        if not isinstance(body, IDesignBody):
            return None
        
        # Update the attribute
        body.GetMaster().SetTextAttribute("MyCustomProp", value)
        
# All DocObjects allow you set Text and Number attributes.  
# We will use these attributes to store our custom property
for body in GetRootPart().GetAllBodies():
    body.GetMaster().SetTextAttribute("MyCustomProp", "This is my description")
 
Application.AddPropertyDisplay(CustomPropertyDisplay("My_custom_properties", "my_title"))