<?php

namespace Stemcell\Customer\Observer;

use Magento\Customer\Api\Data\CustomerInterface;
use Magento\Framework\App\RequestInterface;
use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;

class SalespersonsIdSave implements ObserverInterface
{

    public function execute(Observer $observer)
    {
        /* @var $customer CustomerInterface */
        $customer = $observer->getData('customer');
        /* @var $customer RequestInterface */
        $request = $observer->getData('request');

        $data = $request->getParam('customer');
        if (!empty($data['extension_attributes']['salespersons_id'])) {
            $customer->getExtensionAttributes()->setSalespersonsId(implode(',', $data['extension_attributes']['salespersons_id']));
        }
    }
}