I am attempting to set custom Customer Metafields upon checkout. I've created the metafields and would like data that Shopify is already collecting to be populated in these fields at checkout. We're looking to set UTM_Campaign and UTM_Source as these Metafield attributes. We are working with ShopifyPLUS, so we have access to checkout.liquid, but I'm having a difficult time finding a starting point. Can anyone provide a bit of assistance? Has anyone done something similar?