Add "required" attribute to HTML input element

backend/templates/index.html.template

@@ -60,7 +60,7 @@
     <h1>Shorten URL</h1>
 <form method="post">
       <label for="url">URL to shorten:</label>
-      <input type="url" name="url" placeholder="https://example.com" value="{{url}}">
+      <input type="url" name="url" placeholder="https://example.com" required value="{{url}}">
       <input type="submit" value="Shorten!">
     </form>
     {{shorturl}}