TokenUserContext.php

<?php
public function afterGetUserId(\Magento\Webapi\Model\Authorization\TokenUserContext $subject, $result)
{
    /** @param Request $requestHeader **/
    $integration = $this->requestHeader->getHeader('custID');

    if (!empty($integration)) {
        static $customer = null;

        if ($customer) {
            return $customer->getId();
        }
        // Get customer account in Magento by integration ID.
        $customer = $this->accountManagement->getByMagento($integration);

        if (!empty($customer)) {
            return $customer->getId();
        }
    }
    // Return default context ID.
    return $result;
}