JSFiddle - React, Tailwind, and code Playground

HTML

<div data-role="popup" id="signupPopup" data-theme="a" class="ui-corner-all" data-dismissible="false" data-add-back-btn="true" data-transition="pop">
    <h3 class="centerText">Login</h3>
    
    <div class="popup">
        <form>				
            <input type="email" name="email" id="email" class="centerText" placeholder="email" data-theme="a"/>
            <input type="text"  name="zip"   id="zip"   class="centerText" placeholder="postcode (first part will do)" data-theme="a"/>		    	  	
        </form>
    </div>
</div>

JavaScript

$(document).ready(function(){
    $("#signupPopup").popup('open');
});