example.php

<?php //Please do not include the opening PHP tag if you already have one 

add_filter( 'FHEE__EE_Export__report_registrations__reg_csv_array', 'espresso_add_course_id_csv', 10, 2);
function espresso_add_course_id_csv( $reg_csv_array, $reg_row ) {
    // get the event's ID
    $event_id = $reg_row[ 'Registration.EVT_ID' ];

    //Create the empty 'Course ID' column in the CSV
    $reg_csv_array[ 'Course ID' ] = '';

    // Pull the post mate value
    $my_course_id = get_post_meta( $event_id, 'my_course_id', true );
    
    //Check $mv_course_id has a value
    if ( $my_course_id != '' ) {
        //We have a value, assign it to the empty column we just created.
        $reg_csv_array[ 'Course ID' ] = $my_course_id;
    }

    //Return the CSV row
    return $reg_csv_array;
}