Need help with user.register.post.post_process script

You could get the user data:

user = event.request.body.record; (this is the body of the initial request, you have access to it)
var user_data = _.first(platform.api.get('my-db/users?fields=name,email,city,information', {
        filter: "email = " + user.email
    }).record);

then use it:

platform.api.post('email', {
"username": user_data.name,
"city": user_data.city,
"information": user_data.information,
"template": "My Template Name",
  "to": [
    {
      "name": user_data.name,
      "email": user_data.email
    }
  ]
});