Can you explain more what you need the button to do exactly?
To add a button you would need to customize the source code but if you can tell us exactly what you need to do we may be able to come up with a better solution for you.
The Template itself can use HTML and CSS. Project Settings > Customer Notifications > Templates (Edit Templates). However, it does call the specific message content that you put in plain text in the specific Request created event.
If the button is something you would want on all messages to your customers then you can do it this way, otherwise it is not possible.
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