Tamper Routing MatchResults from a Flow HTTP Component

DetectSubdomainComponent.php

<?php
namespace Your\Package;

use TYPO3\Flow\Annotations as Flow;
use TYPO3\Flow\Http\Component\ComponentContext;
use TYPO3\Flow\Http\Component\ComponentInterface;
use TYPO3\Flow\Mvc\Routing\RoutingComponent;
use TYPO3\Flow\Utility\Arrays;

class DetectSubdomainComponent implements ComponentInterface
{

    /**
     * @param ComponentContext $componentContext
     * @return void
     */
    public function handle(ComponentContext $componentContext)
    {
        $originalMatchResults = $componentContext->getParameter(RoutingComponent::class, 'matchResults');

        // detect subdomain...

        $newMatchResults = Arrays::arrayMergeRecursiveOverrule($originalMatchResults, ['shop' => 123]);
        $componentContext->setParameter(RoutingComponent::class, 'matchResults', $newMatchResults);
    }
}

Settings.yaml

TYPO3:
  Flow:
    http:
      chain:
        process:
          chain:
            'detectSubdomain':
              position: 'after routing'
              component: 'Your\Package\DetectSubdomainComponent'