Template for adding new key:value pair (property) to your AWS API Gateway Body Mapping Template

Add API stage as key, value (property) to POST data in API Gateway

{
    "stage" : "$context.stage",
    #foreach( $key in $input.path('$').keySet() )
        "$key": "$input.path('$').get($key)"#if($foreach.hasNext),
        #end 
    #end
}

Add new key, value (property) to POST data in API Gateway

{
    "edited": "yes",
    #foreach( $key in $input.path('$').keySet() )
        "$key": "$input.path('$').get($key)"#if($foreach.hasNext),
        #end 
    #end
}