Sitecore 9.0.1 Custom form field element is not saving

I have created a custom hidden field element in sitecore 9.0.1 forms(refered sitecore documentation https://doc.sitecore.com/developers/90/sitecore-experience-management/en/walkthrough--creating-a-custom-form-element.html).

I have created a form and added Default form save action. While posting form data to database,the custom field is not saved in database (note: others fields are saved).

Is there anything that I missed ?