user-input.php

<?php
function getUserInput($userId)
{
  $form = JForm::getInstance('user-field', '<form> </form>');
  $element = new SimpleXMLElement('<field />');
  $element->addAttribute('type', 'User');
  $element->addAttribute('name', 'user_id');
  $element->addAttribute('class', 'readonly');
  
  $form->setField($element);
  $form->setValue('user_id', null, $userId);
  
  $field = $form->getField('user_id');
  
  echo $field->input;
}