You need to use this property type in conjunction with another property type (widgets since 8.7, action since 8.9)
Here is the detail from the docs (https://docs.mendix.com/apidocs-mxsdk/apidocs/client-apis-for-pluggable-widgets#listvalue):
"However it is not possible to access domain data directly from ListValue, as every object is represented only by GUID in the items array. Instead, a list of items may be used in combination with other properties, for example with a property of type widgets. The dataSource attribute on that property should reference a datasource property."
Hope that helps