OneSignal's API at https://api.onesignal.com sends push notifications, email, and SMS to your app's users.
Copy your app's REST API key from Settings → Keys & IDs in OneSignal and send it as Authorization: Key <key>. In the examples it's written {{ONESIGNAL_API_KEY}}, and {{ONESIGNAL_APP_ID}} is your app id.
Full reference: https://documentation.onesignal.com/reference/rest-api-overview
included_segments targets a segment; use one targeting method per message.
curl -X POST https://api.onesignal.com/notifications \
-H "Authorization: Key {{ONESIGNAL_API_KEY}}" \
-H "Content-Type: application/json" \
-d '{"app_id": "{{ONESIGNAL_APP_ID}}", "included_segments": ["Total Subscriptions"], "contents": {"en": "Hello from PostTaco"}}'