What have you tried so far to achieve this? Stack Overflow is meant for developers who are actively working through problems and need help refining or debugging their code. Not just a place to request ready-made solutions. It's important to show some effort and experimentation on your part.

Maybe you can take a look at some related questions:
- Create custom Notice-Type, and use it on specific actions(i.e add to cart button)/pages in woommerce (This is a non AJAX solution that involves only PHP - and the default WooCommerce notice behaviar)
- How to display a message on the checkout page after clicking on the proceed to payment button? (This gives you more information about showing messages on custom places like the checkout page)

Or you can create your own modal: https://www.w3schools.com/howto/howto_css_modals.asp
And place your code inside a WooCommerce theme override or action. You can find more about template overrides and inserting your own code in the WooCommerce documentation:
https://developer.woocommerce.com/docs/template-structure-overriding-templates-via-a-theme/

We're happy to help if you run into any code-related issues after trying some of the above solutions.