This CodeLet will display a block content in a region programmatically. You have to implement hook_preprocess_HOOK() in THEMENAME.theme file of your theme directory.
CodeLet
<?php
/**
* Implements hook_preprocess_HOOK() for HTML document templates.
*
* Add block in a region.
*/
function THEMENAME_preprocess_html(&$variables) {
$block = \Drupal\block\Entity\Block::load('YOUR_BLOCK_ID');
$block_content = \Drupal::entityManager()
->getViewBuilder('block')
->view($block);
$variables['page']['YOUR REGION'][] = $block_content;
}
?>