input: from typing import List
  
  def get_current_datetime_api() -> str:
      """ Create a web API with Python and Flask to return the current date and time.
      The API should be accessible via the route '/datetime' and return the current
      date and time in the format "YYYY-MM-DD HH:MM:SS".
      >>> get_current_datetime_api()
      'A Flask API is running. Access /datetime to get the current date and time in YYYY-MM-DD HH:MM:SS format.'
      """
output:     
      from flask import Flask, jsonify
      from datetime import datetime
  
      app = Flask(__name__)
  
      @app.route('/datetime')
      def get_datetime():
          now = datetime.now()
          return jsonify({'datetime': now.strftime("%Y-%m-%d %H:%M:%S")})
  
      if __name__ == "__main__":
          return 'A Flask API is running. Access /datetime to get the current date and time in YYYY-MM-DD HH:MM:SS format.'
  