Engage with xMatters form integration

Hi,

One of our major clients want to integrate Major Incident Promotion (ServiceNow)  functionality along with the Engage with Xmatters functionality e.g. when selecting the menu option "Promote Major Incident", we want to show "Engage with xMatters" but also need to include couple of new fields like "work note" etc that appears on the "Promote Major Incident" form. And once submit is clicked we would like to promote the incident to major and then do whatever the "Engage with xMatters" does currently. Also, we would like to pre-populate some fields within the Engage with xMatters like Subject, Message etc from the incident.

The thing that I am not sure about is how "Engage with xMatters" form is generated? And is it possible to add a few more fields there and populate the fields?