How to Add a Custom field in checkout - Magento module

We had a client who wanted to add some selectable options on the checkout review page, asking their customers how they heard about their website. We created a custom module to make it happen, and it was a big hit! The options included Social Media, Blog, Search Engines, and more. This module adds a custom field to the checkout review section, which can be super useful for understanding how customers find your website. If you’re a developer, you can easily tweak this module to suit your needs. 

Please check the screenshots

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Please find it here in gitHub –https://github.com/hatslogic/module-referrer