CustomForm.php

<?php
namespace MyVendor\MyExt\Form;

class CustomForm extends \FluidTYPO3\Flux\Form {
 
 /**
  * Creates objects inserted into this Form, resulting in
  * a nested set of PHP objects that correspond exactly
  * to what would come out of parsing a Flux template
  */
 public function initializeObject() {
   $this->createField('Input', 'test')->setDefault('default value')->setRequired(TRUE);
 } 
  
}

CustomProvider.php

<?php
namespace MyVendor\MyExt\Provider;

class CustomProvider extends \FluidTYPO3\Flux\Provider\ContentProvider implements \FluidTYPO3\Flux\Provider\ProviderInterface {
  
  /**
   * Gets a special Form instance, if one applies, to be returned
   * instead of the basic Flux Form instance
   */
  public function getForm(array $row) {
    $templateFile = $this->getTemplatePathAndFilename($row);
    switch (basename($templateFile)) {
      ...
      case 'whatever.html':
        $form = $this->objectManager->get('MyVendor\MyExt\Form\CustomForm');
        break;
    }
    return $form;
  }
  
}