Form configuration - add buttons in footer

SSantos You can add button in list form footer like: 

{
    "elmType": "button",
    "txtContent": "Click"
}

But, if you are trying to use "setValue" in form formatting, I don't think it is supported. I will try to get confirmation from list formatting repository maintainers just to be sure. 

Documentation: configure the list form 

In this case, you can customize list form using Power apps to add button & update form values.

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