JSFiddle - React, Tailwind, and code Playground

HTML

<b>*Gebruikersnaam:</b>
<br>
<input id="textbox" type="radio" value="" size="25" required onblur="validateTextBox()">
<a id="googleLink" href="##klik nu om member te kopen##" onclick="this.href='https://websitelink.com?naam=' + encodeURIComponent(document.getElementById('textbox').value);">
  <button>Koop: MEMBER</button>
</a>

JavaScript

function validateTextBox() {
    if (document.getElementById("textbox").value != "") {} else {
        alert("Please enter a value");
    }
}