VTL Mapping Template To Convert Encoded Form Data Into JSON Object For AWS Step Function

How to convert application/x-www-form-urlencoded data coming via POST request into API gateway into a JSON object so you may send said object to Step Functions without first sending to Lambda for pre-processing.

(Answer included below)

1. Inside of your API, go to Integration Request > Edit Integration Request
2. Scroll down to mapping templates
3. Set content type to application/x-www-form-urlencoded
4. Paste the following mapping template below, be sure to update the Step Function ARN appropriately, and be sure your IAM role has valid permissions to StartExecution/StartSyncExecution

Mapping Template:

#set($data = "")

#foreach( $token in $input.path('$').split('&') )
#set( $keyVal = $token.split('=') )
#set( $keyValSize = $keyVal.size() )
#if( $keyValSize >= 1 )
    #set( $key = $util.urlDecode($keyVal[0]) )
    #if( $keyValSize >= 2 )
        #set( $val = $util.urlDecode($keyVal[1]) )
    #else
        #set( $val = '' )
    #end
#end


#set($data = 
"${data}\""${key}\"":\""$util.escapeJavaScript($val)\""#if($foreach.hasNext),#end")
#end


{
  "input": "{$data}",
  "stateMachineArn": "YOUR STEP FUNCTION ARN"
}

Given your role has proper permissions, this mapping template will result in your step function executing properly with a beautifully formatted JSON object inputted.