Hi @Eyal,
At this time, we do not support JavaScript string methods for table format or aggregation manipulations.
The correct approach within the table format is to use data model field names exactly. You cannot extract the first initial from the firstName field using that method at this time. The data must exist already in a field in the Agent table if you would like to display it in the table format.
If you wish, you can request a new feature to add support for it in the New Feature Requests category!