1. Add the channel under Account settings > Notifications
Register a Discord webhook, a Microsoft Teams webhook, an Opsgenie API key, or a Zapier / generic JSON hook, then use Send test so the channel shows as verified. Slack, PagerDuty and webhook endpoints are built in and configured on the same page; they need no registry entry. SMS, voice and mobile push are listed but have no provider yet, so they cannot page anybody.
2. Open the escalation policy and choose what each step pages
Under Account settings > On-call, open a policy and add a step. The picker offers three kinds: a person, one of your notification channels, or a built-in channel (Slack, PagerDuty, all webhook endpoints). An existing step can be changed with Change target. What the chosen target actually carries is written under it: a Discord, Teams or Slack post does not wake anyone unless that app does; Opsgenie and PagerDuty open their own alert under an escalation key and are not closed by acknowledging here.
3. Let each person say how to page them
A step that names a person always emails them. On the Profile page, under How to page me, that person can also tick any of the account's notification channels, and every step that reaches them (directly, or as whoever is on call in a rotation) pages those channels too. Email cannot be switched off, so a page is never silent.
4. Acknowledge from the dashboard
Escalation stops the moment somebody acknowledges the incident on the RealUptime dashboard, whatever channel they read it on. A step whose channel is disabled or whose built-in is not set up pages nobody for that step and the ladder keeps moving; the editor says so next to the step.
Go deeper
The full reference lives in the docs: Alerting and on-call reference. Error codes named above are each explained in the error-code reference.