While registring a new Machine with static templates via MQTT I couldn't find a way to put something into the "Notes" Field at Device Profile in the Device Info.
Is there a template or another way to put some additional Information in that Textbox?
There is no static template for setting the notes but you can create your own inventory PUT template. The fragment for this text box is simply c8y_Notes which is a string.
To use the template you would send:
123,<serialOfDevice>,"My device notes"
I recommend to use quotes for longer strings to handle line breaks etc. correctly.