When a cook cycle is finished, have Alexa make an announcement.

TSmartLife app provides enhanced capabilities for Toshiba branded Smart Home appliances, you can connect to make full use of Toshiba Smart Products! (Kindly notice: Due to the safety requirement, if microwave is in standby state, please reclose the door, and then it can be controlled remotely within 10 minutes.)
Create custom workflows effortlessly by selecting triggers, queries, and actions. With IFTTT, it's simple: “If this” trigger happens, “then that” action follows. Streamline tasks and save time with automated workflows that work for you.
4 triggers, queries, and actions