Magento 2 Checkout Custom Field

I have a Magento 2 store built which is nearly ready for launch and we wish to launch as soon as possible. The one thing we need to add is a single custom field at checkout. The field takes text input and is a required field. The text entered needs to be part of the order record as we need the information stored with every order. The custom field needs to be implemented alongside our existing build and be fully tested so that all of our existing working features continue to work. We can do final testing, but you should do basic testing. Our existing build uses Magento 2, Magentech SM Market Theme and Webkul Extensions - Marketplace, Gift Cards & Reward Points. As a note, we did purchase the BSS Commerce Custom Field Extension and it interfered with the checkout screen making the customer have to place the order, refresh the screen and then place the order again, so we are not going to use that extension. To emphasise, we need this to be done fast and we need it to work perfectly.