Magento2 Create account custom field form server-side validation

I added a custom sponsor name field in the magento2 account creation form and also validated the client side. How can I do server-side validation? Need to consult sponsor is available in the database or not.

    
escapeHtml (__ (& # 39; Sponsor Information & # 39;))?>
    
    

        
        
        
        <input type = "text" name = "sponsor_name" id = "sponsor_name" title = ""class =" input-text required-entry validate-length minimum-length-5 v-validate "autocomplete =" off "value =" ">