From https://eventespresso.com/topic/posting-registration-url-to-facebook/#post-189425 Sync EE3 featured image from event to blog post

functions.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

/* syncs EE3 event featured image to event's blog post */

add_action( 'action_hook_espresso_insert_event_success', 'my_ee3_event_image_function' );
add_action( 'action_hook_espresso_update_event_success', 'my_ee3_event_image_function' );
function my_ee3_event_image_function( $event_data ) {
	global $wpdb;
	if ( isset( $event_data['upload_image'] ) && ! empty( $event_data['upload_image'] ) ){
		$image_url = $event_data['upload_image'];
		$image_url = preg_replace( '/-\\d+[Xx]\\d+\\./', '.', $image_url );
		$attachment = $wpdb->get_col( $wpdb->prepare( "SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url ) );
		if ( ! empty( $attachment ) ) {
			$attach_id = $attachment[0];
			$post_id = $event_data['post_id'];
			set_post_thumbnail( $post_id, $attach_id );
		}
	}
}