instantiate_tool_on_resource.php

<?php

public function fields(Request $request)
{ 
   return [
      // ... All your other fields here

      GroupByTool::make()
            ->usingModel(EventRegistration::class)
            ->selectColumns([
                "shirt_size",
                "COUNT('shirt_size') as total_count",
            ])
            ->groupBy(['shirt_size'])
            ->usingWhere([
                'eventable_type' => \App\Models\StandaloneEvent::class,
                'eventable_id'   => $this->id,
            ])
            ->setTitle('Count by T-Shirt Sizes'),
     
     ];
}