In Xamarin.Android, if you want to show heads-up notification, you have to set the priority to High or Max: builder.SetPriority (NotificationPriority.High);
According to the source of Notifier plugin, there is no interface for the user to set the priority, so it does not support heads up notification.
If you really want to use it, I can suggest 2 solutions for you:
1) Post an issue to the author of Notifier Plugin to modify it to support heads-up notification.
2) Create a notification yourself in Xamarin.Android instead of using the plugin.
And in your code, you can write this:
builder.SetPriority (NotificationPriority.High);
or
builder.SetPriority (NotificationPriority.Max);
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