Magento2 Create account form custom field server side validation

Question

I Added a custom sponsor name field on magento2 account creation form and done client side validation also. How can I do server side validation??? Need to check sponsor is available in database or not.

<div class="field required">
    <legend class="legend"><span><?php echo $block->escapeHtml(__('Sponsor Information')) ?></span></legend>
    <label for="sponsor_name" class="label"><span><?php /* @escapeNotVerified */ echo __('Sponsor Name') ?></span></label>
    <div class="control">
        <input type="text" name="sponsor_name" id="sponsor_name" title="<?php /* @escapeNotVerified */ echo __('Sponsor name') ?>" class="input-text required-entry validate-length minimum-length-5 v-validate"  autocomplete="off" value="" >
        <div class="sponsor-msg-div"></div>
    </div>
</div>

Answer 1

i had done validation using plugin before create account execute function.

..\etc\di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Customer\Controller\Account\CreatePost">
        <plugin name="Kblikruta_Customerattr_Plugin" type="Kblikruta\Customerattr\Plugin\Checkattribute" sortOrder="10" disabled="false"  />
    </type>
</config>

..\Plugin\Checkattribute.php

namespace Kblikruta\Customerattr\Plugin;

use Magento\Framework\Controller\ResultFactory;

use Magento\Framework\Message\ManagerInterface;

use Magento\Framework\App\RequestInterface;


class Checkattribute
{

    public function __construct(
        ResultFactory $Redirect, 
        ManagerInterface $messageManager,   
        RequestInterface $request
    )
    {
        $this->resultFactory = $Redirect;
        $this->_messageManager = $messageManager;
        $this->getRequest = $request;
    }

    public function beforeExecute(\Magento\Customer\Controller\Account\CreatePost $subject)
    {

        $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $customerObj = $objectManager->create('Magento\Customer\Model\ResourceModel\Customer\Collection');
        $collection = $customerObj->addAttributeToSelect('*')
                  ->addAttributeToFilter('sponsor_name',$this->getRequest->getParam('sponsor_name'))
                  ->load();
        $customerdata=$collection->getData();
        if(!empty($customerdata))
        {
            $resultRedirect = $this->resultFactory->create(ResultFactory::TYPE_REDIRECT);
            $this->_messageManager->addError('The value of "Sponsor Name" already exist.');
            $resultRedirect->setPath('customer/account/');
            $this->getRequest->setParam('form_key','');
            return $resultRedirect;
        }
        else
        {
            return true;
        }

    }

}

Answer 2

Unfortunately I cannot comment, so I will try a response. The solution proposed by snkt worked but it does not stop the form from executing and adding the customer. Do you know how can I add the error message and also prevent from adding the customer ?

Thanks !

================

I added the following line to solve it if the validation returns false:

        $this->getRequest->setParam('email_address','');