Add custom fields depending on the category

What I try to do is to create a custom field that only show up if a specific event categorie has been used.

I’m having a really hard time following the docs at Custom Field API. So far I found out that you would need to create a folder in /wp-content/plugins/<new folder> and create a new php file. Afterwards you need to activate this plugin in wordpress.

Here is my code so far (currently all fields are set to global):

<?php
/*
Plugin Name: My Tickets: Custom Fields
Plugin URI: https://www.joedolson.com/my-tickets/
Description: Custom Fields in My Tickets
Version: 1.0.0
Author: Daniel Klauck
Author URI: https://www.joedolson.com/
*/
/*  Copyright 2014-2018  Joseph C Dolson  (email : [email protected])

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

add_filter( 'mt_custom_fields', 'create_custom_fields', 10, 1 );
	function create_custom_fields( $array ) {
		// Other fields: sanitize callback; input type; input values; display_callback
		$array['message_to_organizer'] = array( 
			'title'=>"Nachricht an den Veranstalter:", 
			'sanitize_callback'=>'sanitize_callback', 
			'display_callback'=>'display_callback',
			'input_type'=>'input',
			'context'=> 'global'
		);
		$array['motocycle_type'] = array( 
			'title'=>"Welches Motorrad f?hrst du?:", 
			'sanitize_callback'=>'sanitize_callback', 
			'display_callback'=>'display_callback',
			'input_type'=>'input',
			'context'=> 'global'
		);
		$array['drivers_experience'] = array( 
			'title'=>"Meine Off Road Erfahrung:", 
			'sanitize_callback'=>'sanitize_callback', 
			'display_callback'=>'display_callback',
			'input_type'=>'select',
			'input_values'=>array( 'keine Off Road Erfahrung', 'wenig Off Road Erfahrung', 'viel Off Road Erfahrung' ),
			'context'=> 'global'
		);
		return $array;
	}

	/* Display callback formats the saved data. $context is 'payment' or 'cart', depending on whether it appears in an admin payment or the user's shopping cart. */
	function display_callback( $data, $context='payment' ) {
		return urldecode( $data );
	}

	/* Sanitize callback cleans the data before saving to the DB */
	function sanitize_callback( $data ) {
		return esc_sql( $data );
	}
?>

I would like to set “motocycle_type” and “drivers_experience” to only show up if the event category is set to “Off-Road Trainings”.

The page I need help with: [log in to see the link]