https://domain.com/index.php/rest/V1/integration/customer/token?username=test@gmail.com&password=12345

protected function getCustomerToken($emailId){
 /**
* @var MagentoCustomerModelCustomer $customer */
*/
$customer->loadByEmail($emailId);
if($customer->getId()){
        /**
        * @var MagentoIntegrationModelOauthTokenFactory $tokenModelFactory
        */
        $customerToken = $this->tokenModelFactory->create();
        $tokenKey = $customerToken->createCustomerToken($customerId)->getToken();
        return $tokenKey;
}
return "YOU MSG FOR CUSTOMER NOT FOUND";
}

/**
 * {@inheritdoc}
 */
public function createCustomerAccessToken($username, $password)
{
    $this->validatorHelper->validate($username, $password);
    $this->getRequestThrottler()->throttle($username, RequestThrottler::USER_TYPE_CUSTOMER);
    try {
        $customerDataObject = $this->accountManagement->authenticate($username, $password);
    } catch (Exception $e) {
        $this->getRequestThrottler()->logAuthenticationFailure($username, RequestThrottler::USER_TYPE_CUSTOMER);
        throw new AuthenticationException(
            __('You did not sign in correctly or your account is temporarily disabled.')
        );
    }
    $this->getRequestThrottler()->resetAuthenticationFailuresCount($username, RequestThrottler::USER_TYPE_CUSTOMER);
    return $this->tokenModelFactory->create()->createCustomerToken($customerDataObject->getId())->getToken();
}

<?php


namespace TestModuleControllerTest;

use MagentoCustomerModelSession;
use MagentoFrameworkAppActionContext;


class Token extends MagentoCustomerControllerAbstractAccount
{
    /**
    * @var MagentoCustomerModelSession
    */
    protected $_customerSession;

    public function __construct(
        Context $context,
        Session $customerSession,
        MagentoIntegrationModelOauthTokenFactory $tokenModelFactory
    ) {
        $this->_customerSession = $customerSession;
        $this->_tokenModelFactory = $tokenModelFactory;
        parent::__construct(
            $context
        );
    }

    public function execute()
    {
        $customerId = $this->_customerSession->getCustomer()->getId();
        $customerToken = $this->_tokenModelFactory->create();
        echo "Customer-token=> ".$tokenKey = $customerToken->createCustomerToken($customerId)->getToken();
    }
}

class AutoLoginManagement implements FloCoreApiAutoLoginManagementInterface
{

    protected $_customer;
    protected $_customerSession;
    protected $_tokenModelFactory;

    public function __construct(
        MagentoIntegrationModelOauthTokenFactory $tokenModelFactory,
        MagentoCustomerModelCustomer $customer,
        MagentoCustomerModelSession $customerSession
    )
    {
        $this->_customer = $customer;
        $this->_customerSession = $customerSession;
        $this->_tokenModelFactory = $tokenModelFactory;
    }

    public function postAutoLogin($data)
    {

        $objectManager = MagentoFrameworkAppObjectManager::getInstance();
        // Load customer
        $customer = $objectManager->create('MagentoCustomerModelCustomer')->load($data['customer_id']);

        if(! $customer->getId()) {
            return 'Not Found';
        } else {
            // Load customer session
            $customerSession = $objectManager->create('MagentoCustomerModelSession');
            $customerSession->setCustomerAsLoggedIn($customer);

            $customerToken = $this->_tokenModelFactory->create();
            $tokenKey = $customerToken->createCustomerToken($customer->getId())->getToken();
            return $tokenKey;

        }

    }

    private function getCustomerToken($customerId)
    {
        $customerToken = $this->tokenModelFactory->create();
        $tokenKey = $customerToken->createCustomerToken($customerId)->getToken();
        return $tokenKey;
    }
}