I've integrated Nextpeer's push notification by following this guide: https://developers.nextpeer.com/docs/view/iospushguide
However, i'm not sure how to enter my push notification's message as it was not stated in the document. Any help is greatly appreciated.
First, I think that you should contact Nextpeer's official support mail (support [at] nextpeer [dot] com). Their support team is very responsive.
In anyway, Nextpeer automates the push notification system for you. You don't need to "send" any message. For example each time your user invites another user, Nextpeer sends a push notification automatically to the invitee.