Add "how did you hear about us" after checkout

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I currently have a "how did you hear about us" drop down on my cart page. I don't like that it is automatically set to be required before a customer can check out and I would like to change it so that this comes up AFTER the customer has placed their order. I've seen this done on other Shopify sites so I know it's possible, but I'm just not sure how to do it. I'm using the Debut theme and my site is