Line-item Scripts are checkout scripts only available to Shopify Plus plans.
Please do not label things arbitrarily.
Your on the right track all it normally takes for a required input is the required attribute
Though in some themes there is a [novalidate attribute on the forms](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/form#:~:text=type%3D%22image%22%3E%20elements.-,novalidate,-This%20Boolean%20attribute) that needs to be removed.
For themes based on the dawn reference theme that should be in buy-buttons snippet [https://github.com/Shopify/dawn/blob/main/snippets/buy-buttons.liquid#L47](https://github.com/Shopify/dawn/blob/main/snippets/buy-buttons.liquid#L47)