uppy/website/src/examples/modal/app.html

<!-- Basic Uppy styles -->
<link rel="stylesheet" href="/uppy/uppy.css">

<!-- Modal trigger -->
<button id="uppyModalOpener" class="UppyModalOpenerBtn">Open Uppy Modal</button>

<script>
  window.addEventListener('load', function () {
    if ( localStorage.getItem('openUppyModalOnLoad') ) {
      document.querySelector('#uppyModalOpener').click()
    }
  }, false);
</script>