Our theme has been coded to work straight away with Mailchimp. It doesn’t have any native compatibility with any other responder.
You may wish to investigate if there is a third-party plugin that can link your website to your email provider.
Here are the list of free plugin some of our clients use:
- Aweber -> https://wordpress.org/plugins/aweber-web-form-widget/
- Active Campain -> https://en-gb.wordpress.org/plugins/activecampaign-subscription-forms/
- ConvertKit -> https://en-gb.wordpress.org/plugins/convertkit/
You can setup your form with this plugin then, in the opt-in options, simply replace the shortcode by the shortcode given by the plugin.
Bare in mind you may have to do your own CSS to make it look like the opt-in form from the demo.