I don’t think there is an option specify multi-valued for application extended attributes.
Check whether your use case can be met by storing the value in comma separated format.
in the beanshell whenever you want to take the list object you can first get reference to the rule and then execute in in the beanshell to get the list as a result into your variable.
After extensive research, it appears that making the Application Extended attribute multivalued is not currently supported. Unfortunately, it seems that the only viable approach at the moment is to manually add comma-separated values.
While this may not be the ideal solution, it is the most practical option available given the current limitations. If any advancements or alternative methods emerge in the future, it would be beneficial to explore those options.