Alter Apache Solr Query and Boost Results

custom_search.services.ymlcustom_search.services.yml

services:
  custom_search.event_subscriber:
    class: Drupal\custom_search\EventSubscriber\CustomSearchSubscriber
    tags:
      - { name: event_subscriber }

CustomSearchSubscriber.php

<?php declare(strict_types = 1);

namespace Drupal\custom_search\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\RequestEvent;
use Symfony\Component\HttpKernel\Event\ResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Drupal\search_api_solr\Event\PreQueryEvent;
use Drupal\search_api_solr\Event\SearchApiSolrEvents;
use Drupal\search_api\Query\QueryInterface as SapiQueryInterface;
use Solarium\Core\Query\QueryInterface as SolariumQueryInterface;
/**
 * @todo Add description for this subscriber.
 */
final class CustomSearchSubscriber implements EventSubscriberInterface {

  /**
   * Kernel request event handler.
   */
  public function onKernelRequest(RequestEvent $event): void {
    // @todo Place your code here.
  }

  /**
   * Kernel response event handler.
   */
  public function onKernelResponse(ResponseEvent $event): void {
    // @todo Place your code here.
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents(): array {
    return [
      SearchApiSolrEvents::PRE_QUERY => 'preQuery',
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function preQuery(PreQueryEvent $event): void {
     
    $query = $event->getSearchApiQuery();
    
    $solarium_query = $event->getSolariumQuery();
    // change tm_X3b_en_title and ss_field_order field names as per you solr field names.
    // Searching the text "testing" in tm_X3b_en_title field and boosting the ss_field_order field
    // having the value as "first".
    $solarium_query->addParam("q",  "(tm_X3b_en_title:testing AND ss_field_order:first^5)");
 }

}