Add Custom Data to popoover

functions.php

/**
 * Add PDF Link to Calendar Popover
 */
add_filter( 'vczapi-pro-calendar-extended-props', 'codemanas_add_calendar_additional_fields', 10, 2 );
function codemanas_add_calendar_additional_fields( $extendedProps, $meeting_id ) {
	$pdf_link                        = get_post_meta( $meeting_id, 'vczapi_meeting_pdf_link', true );
	$pdf_html                        = ! empty( $pdf_link ) ? '<div class="pdf-link"><a href="' . $pdf_link . '" target="_blank" rel="nofollow noopener">Things to read before the meeting</a></div>' : '';
	$extendedProps['meetingPDFLink'] = $pdf_html;

	return $extendedProps;
}