Choose Speak up, pick what it's about, and write your message. It goes to your line manager, who sees it word for word, and knows it came from your team, but not who sent it. They can pass it to overall management.
If you like, answer any dropdowns your managers have added, such as your location.
Managers can reply without knowing who you are, and they see your message and any reply together. Their reply shows their name and title. You'll see it under My voice, on the device you wrote it from, and you can answer anonymously. Discord can't notify you, because that would mean knowing who you are. Sensa isn't for emergencies or formal complaints, so please follow your organization's process for those.