To programmatically submit a form and still trigger a "submit" event, besides using .requestSubmit(), would be to use .click() method on the "submit" button. In the example below are the following:

• A <form> that will post it's data to a live test server. The test server's response will be displayed in an <frame> located after the <form>.

• A <button> outside of the <form> and has no form attribute assigned to it either. This button is isolated from the <form> but it will still trigger a "submit" event indirectly since it is registered to the "click" event and the event handler will programmatically click the "submit" button.

• Note, the <button> is not necessary to use the .click() method, it can be invoked by other means like any other method or function.

Click the Trigger button

<button class='trigger'>Trigger</button>

const trigger = document.querySelector('.trigger');

trigger.onclick = e => document.querySelector('form button').click();

.trigger {
  vertical-align: top
}

<form id='UI' action='https://httpbin.org/post' method='POST' target='response'>
  <fieldset>
    <legend>Programmatically Submit a Form</legend>
    <input name='test' value='TEST'>
    <button name='send'>Send</button>
  </fieldset>
</form>
<hr>
<button class='trigger'>Trigger</button>
<iframe name='response'></iframe>