Recent improvements have been made to the way that Outages can be recorded in ServiceNow from resolved P1 & P2 Incidents and approved Changes.
In both cases by right clicking on the resolved Incident & approved Change Request form header bars and selecting Create Outage, you are taken straight to the Outage form with all relevant fields populated with the required information.
At this point, you should check that the outage type and information in the fields is correct and for P1 & P2 Incidents ensure the Problem Record number field is populated as well as the Incident/Change number field.
Once satisfied the information is correct you should click the Submit button to save the Outage record and generate the notification email.