How to alter view query after exposed filter is changed?
Using d70rr3s suggestion and this patch (required), I came across this solution that worked. 

use Drupal\views\ViewExecutable;
use Drupal\views\Plugin\views\query\QueryPluginBase;

/**
 * Implements hook_views_query_alter().
 */
function mymodule_views_query_alter(ViewExecutable $view, QueryPluginBase $query) {
   if ($view-&gt;id() == 'myview'  &amp;&amp; !empty($view-&gt;exposed_raw_input['field_country']) &amp;&amp; $view-&gt;exposed_raw_input['field_country'] !== 'all') {
    // Traverse through the 'where' part of the query.
    foreach ($query-&gt;where as &amp;$condition_group) {
      foreach ($condition_group['conditions'] as &amp;$condition) {
        if ($condition['field'] == 'your_entity.field_country') {
          $field = 'node__field_country.field_country_target_id';
          $value = 'somevalue';
          $operator = '=';
          $query-&gt;addWhere(2, $field, $value, $operator);
        }
      }
    }
  }
}


Results:

WHERE (((node__field_country.field_country_target_id = '10')) OR (node__field_country.field_country_target_id = 'someValue')) AND (node_field_data.status=1) AND (node_field_data.type='article')


Filter Criteria where #1 and #2 from screenshot had to be applied: