JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form22" action="javascript: alert('Form submitted!');" onSubmit="return formValidation(this);">

    <p>
    License Agreement:<br />
    <textarea name="licenseAgreement" rows="10" cols="45">This is my long, long license agreement where you agree to all of my demands.

    This is my long, long license agreement where you agree to all of my demands.

    This is my long, long license agreement where you agree to all of my demands.

    This is my long, long license agreement where you agree to all of my demands.

    This is my long, long license agreement where you agree to all of my demands.

    This is my long, long license agreement where you agree to all of my demands.

    This is my long, long license agreement where you agree to all of my demands.

    This is my long, long license agreement where you agree to all of my demands.

    This is my long, long license agreement where you agree to all of my demands.

</textarea>
    </p>

    <p>
    <input name="button" type="checkbox" value="Submit" disabled> I agree
    </p>

</form>

</body>
</html>

JavaScript

document.getElementsByName("licenseAgreement")[0].addEventListener("scroll", checkScrollHeight, false);

function checkScrollHeight(){
    var agreementTextElement = document.getElementsByName("licenseAgreement")[0] 
    if ((agreementTextElement.scrollTop + agreementTextElement.offsetHeight) >= agreementTextElement.scrollHeight){
        document.getElementsByName("button")[0].disabled = false;
    }
}