You can use the following block code to load a form in a block in Drupal 8.
<?php
namespace Drupal\example_module\Plugin\Block;
use Drupal\Core\Block\BlockBase;
/**
* Provides a 'ExmpleFormBlock' block.
*
* @Block(
* id = "ExampleFormBlock",
* admin_label = @Translation("Example Form Block"),
* )
*/
class ExampleFormBlock extends BlockBase {
/**
* {@inheritdoc}
*/
public function build() {
return \Drupal::formBuilder()->getForm('Drupal\example_module\Form\ExampleForm');
}
}
Example Form is the class of form which has to be displayed in the block. You can also use core and contributed provided forms.