Calendarista Wordpress Admin Code Integration

I need a WordPress developer experienced with Calendarista to create a seamless appointment booking system for our workers. The goal is to allow them to make appointments without entering the WordPress management dashboard, minimizing errors.

Requirements:
- Payment section modification: Add an "Admin Code" field, which can be left blank.
- If the correct admin code is entered at checkout, the system should:
- Skip payment
- Automatically approve the reservation
- If the admin code is incorrect, collect the following information:
- Name
- Phone
- Country
- State
- Send the collected info to a predetermined address
- Mark the selected days as reserved

Ideal Skills & Experience:
- Proficient in WordPress and familiar with Calendarista
- Strong coding skills, especially in modifying payment sections
- Experience with form handling and data submission in WordPress environment

Looking for a developer who can ensure a smooth, error-free implementation of this functionality. Please include relevant experience in your bid.