In Tools & Settings > Services Management, add the ability to restart MySQL
Can you tell what made you need MySQL restart function in Plesk? i.e. if you just reconfigured its setting s in configuration file, isn’t it easier to restart from shell terminal?
UPD: Sorry, we are closing the request as no information were provided for over a month.
This is very tricky, as Plesk relies heavily on MySQL. Stopping/starting MySQL might cause trouble if eg. you have hung queries. What should Plesk do if MySQL does not start? It would break completely.