How can I use variable data in a Notify service call message?

When I make a notification service call, how can I include variable data in the message? I get a variety of error messages and no combination of brackets, prens, or functions I’ve tried works.

For example:
Fan turned on, attic humidity was (value of sensor.sonoff_10004b0fb5_humidity should go here) %

Here is sample notification I use for garbage collection:

- service: notify.mobile_app_mireks_iphone
    title: "UWAGA !!!"
    message: "Dziś odbiór śmieci: {{ states('sensor.garbage_to_collect') }}"