I hope @appmysite will show this how to do. Because their help page is limited about this.
I mean this url:
https://www.appmysite.com/support/kb-category/push-notifications/
There is not all information and we can not puzzle it self, specially when you dont have any experience with firebase… A video from the begining till the end for send notificaiton with firebase will be very helpfull.