public class DemoBlock extends BaseEntityBlock {
    [...]
    @Override
    protected InteractionResult useItemOn(ItemStack stack, BlockState state, Level level, BlockPos pos, Player player, InteractionHand hand, BlockHitResult hit) {
        if (level.isClient) return InteractionResult.SUCCESS;
        
        if (!(level.getBlockEntity(pos) instanceof DemoBlockEntity blockEntity)) {
            return InteractionResult.PASS;
        }

        if (!player.getItemInHand(hand).isEmpty()) {
            // Check what is the first open slot and put an item from the player's hand there
            if (blockEntity.getItem(0).isEmpty()) {
                // Put the stack the player is holding into the container
                blockEntity.setItem(0, player.getItemInHand(hand).copy());
                // Remove the stack from the player's hand
                player.getItemInHand(hand).setCount(0);
            } else if (blockEntity.getItem(1).isEmpty()) {
                blockEntity.setItem(1, player.getItemInHand(hand).copy());
                player.getItemInHand(hand).setCount(0);
            } else {
                // If the container is full we'll notify the player
                player.displayClientMessage(Component.literal("The inventory is full! The first slot holds ")
                    .append(blockEntity.getItem(0).getItemName())
                    .append(" and the second slot holds ")
                    .append(blockEntity.getItem(1).getItemName()));
            }
        } 
        return InteractionResult.SUCCESS;
    }
}