If your goal is to set a value on insert, you don't need a trigger for that. You can just declare that property with an InitialExpression defined.

That is already true as seen in Vitaliy's comment above. Personally, I prefer to use {}.%New() and then use the %Set method because you can provide a third argument to explicitly say whether the value you are adding is a number, string, boolean, stream, etc. to eliminate any ambiguity. We have alphanumeric fields that should be considered strings, but if they happen to be all numbers, they are incorrectly inserted as numeric values in JSON if we don't specify.

You would probably be better served using XPath instead of the XML reader. When you evaluate and XPath expression, it will recognize the CDATA and parse it correctly, returning either a value or another XML object.