TYPO3 Core

    public function initializeFormElement(\TYPO3\CMS\Form\Domain\Model\Renderable\RenderableInterface $renderable)
    {
        if ($renderable->getUniqueIdentifier() === 'something-else') {

            $renderable->setDefaultValue('foo');
        }
}

renderingOptions:
  submitButtonLabel: Submit
  addQueryString: 'true'
type: Form
identifier: something
label: Something
prototypeName: standard
renderables:
  -
    renderingOptions:
      previousButtonLabel: 'previous Step'
      nextButtonLabel: 'next Step'
    type: Page
    identifier: page-1
    label: ''
    renderables:
      -
        defaultValue: ''
        type: Hidden
        identifier: else
        label: 'Else'

Patch set 1 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/54234

Patch set 1 for branch TYPO3_8-7 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/54235