<event name="controller_action_predispatch">
    <observer name="switch_store" instance="NamespaceModulenameObserverSwitchStore" shared="false" />
</event>

protected $storeManager;

public function __construct(
    MagentoStoreModelStoreManagerInterface $storeManager
) {
    $this->storeManager = $storeManager;
}

public function execute(MagentoFrameworkEventObserver $observer) {
    if ($this->firstVisit()) { // Customer's first visit logic inside firstVisit() function
        $storeId = $this->storeManager->getStore($target_store)->getId();
        $this->storeManager->setCurrentStore($storeId);
    }
    return $this;
}