Inline Scripting - To increment a unique value by +1 each submission?

You can extract the value from the JSON payload, cast it to an Integer, increment it, then re-set it in your JSON payload:

import groovy.json.*

String json // Assuming this your JSON payload, wherever you get it from
def map = new JsonSlurper().parseText(json)

Integer submissionId = Integer.parseInt(map.getAt('whatevs_submissionid')) + 1 // maybe wrap with try/catch here?
map.putAt('whatevs_submissionid', submissionId.toString()) // Put it back in the map

json = new JsonBuilder(map).toPrettyString() // Map back to JSON

Is this what you're trying to accomplish?