function siteseo_extended_schemas_mapping_select($select) {
	//Add the new group option + option to the list
	$select['Custom variables'] = [
		'my_custom_var_key' => __('Your Custom variable', 'siteseo'),
	];

	return $select;
}
add_filter('siteseo_schemas_mapping_select', 'siteseo_extended_schemas_mapping_select');

function siteseo_extended_schemas_dyn_variables($vars) {
	//add the variable key
	$vars[] = 'my_custom_var_key';
	
	return $vars;
}
add_filter('siteseo_schemas_dyn_variables', 'siteseo_extended_schemas_dyn_variables');

function siteseo_extended_schemas_dyn_variables_replace($values) {
	//replace the new key by our own value
	//eg: author last name
	$values[] = get_the_author_meta('last_name');

	return $values;
}
add_filter('siteseo_schemas_dyn_variables_replace', 'siteseo_extended_schemas_dyn_variables_replace');