How can I show a custom popup message in the admin ui while creating an entry if a certain condition met inside beforeCreate hook?

Questions and Answers
2

It’s been a while since the question is asked but still it might help someone else.

First requirement here is that your backend should allow to save the value without constrains.
On the frontend side you can do your validation before saving, and then use showNotification() hook from strapi to send a warning message as a parallel process in your saveHandler.

import { useNotification } from "@strapi/helper-plugin";

...

const YourAwesomeComponent = () => {
  const showNotification  = useNotification();

  const  save = () => {
    if (price > 150000000 || price <1000000 ) {
      // This shows a notification message as a warning
      // @see https://github.com/strapi/strapi/blob/v4.21.1/packages/core/helper-plugin/src/features/Notifications.tsx#L26
      // If you are using newer version of strapi then check the new version of this file, it may be changed over time
      showNotification({ message: "Price must be between 1.000.000 and 150.000.000", title: "Success", type: 'softWarning'});
    }

    // proceed to save logic
  }

  // ...
}

The same way you can use to display success message as well, just by changing the type of showNotification