// Your middleware will only be called when the action_id matches 'select_user' AND the block_id matches 'assign_ticket'
app.action({ action_id: 'select_user', block_id: 'assign_ticket' },
  async ({ action, ack, context }) => {
    await ack();
    try {
      const result = await app.client.reactions.add({
        token: context.botToken,
        name: 'white_check_mark',
        timestamp: action.ts,
        channel: action.channel.id
      });
    }
    catch (error) {
      console.error(error);
    }
  });