Integrate email service with theme using custom PHP POST form submission code?

Hi,

I’m on a non-existent budget right now, so I have to integrate a free email service like Envialosimple or ActiveCampaign with a landing page theme or template similar to JustLanded.

Those two services do have free plugins, but the forms they generate are small and unlikely to be effective. That’s why I want to integrate them into a better-looking landing page template or theme.

Most of the landing page themes and templates I’ve tried can automatically integrate common services like MailChimp and Aweber, but not less common free ones. Some of them have a box in the settings section where you can integrate an unsupported third party email service with a custom integration code, but I can’t figure out how to create this. I think it requires a PHP POST form submission, but that’s the most I’ve been able to learn.

Can you help me create this custom integration code for either Envialosimple or Active Camppaign using a PHP POST form submission, or at least point me in the right direction? I would really appreciate it. Thanks.