How is Contact Form 7 validated?

Question

WordPress

According to the Contact Form 7 standard, validation is done using the .wpcf7-validates-as-required class. When trying to submit a form, a message is displayed in .wpcf7-response-output. But how to make it so that a message is displayed under an empty field about the need to fill in this field, as in the picture?

I see that there is such a message in the settings:

How to make the field have required="required". Can this be done somehow in the theme's functions.php file?
Or how to do your validation before submitting the form data?