modify-value-filed.php

<?php

// Podemos hacerlo en el hook_entity_presave(), ejemplo en un block.

/**
 * Implements hook_entity_presave().
 */
function MODULO_NAME_block_content_presave(BlockContent $entity) {
  $value_title = $entity->field_container_url->title;
  if ($value_title != "") {
    $entity->field_container_url->title = 'Title';
  }
}

/**
 * Implements hook_form_alter().
 */
function MODULENAME_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // Modificamos el valor del campo boletines para que se guarde.
  $form['field_X']['widget'][0]['#element_validate'][] = '_MODULENAME_field_X_validate';
  $form['field_X']['widget'][0]['value']['#required'] = FALSE;
  $form['field_X']['#access'] = FALSE;
}

/**
 * Callback.
 */
function _MODULENAME_field_X_validate(array $element, FormStateInterface $form_state, &$complete_form) {
  $form_state->setValueForElement($element, ['value' => 'VALUE']);
}