Additional custom fields

If you are not a WordPress developer you could consider to use the “Advanced Custom Fields” plugin:
https://nl.www.ads-software.com/plugins/advanced-custom-fields/

If you are familiair with WordPress hooks you can also use the pronamic_event_fields filter from the Pronamic Events plugin:
https://github.com/pronamic/wp-pronamic-events/blob/1.2.1/admin/meta-box-event-details.php#L59-L87

You could for example add the following code/function to a custom plugin or your custom theme functions.php file:

<?php

function my_pronamic_event_fields( $fields ) {
    $fields['organiser'] = array(
        'id'       => 'pronamic_event_organiser',
        'label'    => __( 'Organiser', 'my-text-domain' ),
        'meta_key' => '_pronamic_event_organiser',
    );

    return $fields;
}

add_filter( 'pronamic_event_fields', 'my_pronamic_event_fields' );

Thanks,
rt.