How to get NotifyMyAndroid Actions

How to get the notifications on the andriod mobile. mqtt message sends or sensor is triggered

Install the binding and call the actions within your rules triggered by the MQTT message Item’s being updated as documented here.

Which API I have to activate in the API-Dashboard? In the documentation it is not clear…

You install the NMA Action from the Actions section on PaperUI.

The NMA Action is installed. But I need the API Key either in the command or the .cfg file…
Where I can get this API key? Do I have install any service (screenshot)?

You get the API key from the NMA website. You also have to install the NMA application on to your Android phone.