You can add a Kit form (formerly Convertkit) to your Wix site by using an iframe, but keep in mind that only inline forms tend to work reliably: INLINE FORM

Modal and slide-in forms often won’t load correctly within Wix’s setup. If your form isn’t appearing as expected, it might be necessary to reach out to Wix support and ask them to enable iframe display for third-party forms: Contact Wix

 What You Need:

• A Kit (ConvertKit) account (free or paid)

• A Wix site (Editor or Studio)

Here are the steps to embed your Kit form in Wix:

1. Build your form in Kit: https://app.kit.com/forms
   
   Here's the documentation: How to create your first Form in Kit
   
   
2. Open the form you want to embed and click the "Embed" tab.
   
   
3. Choose "HTML" and use the "Copy" button to copy the form's HTML:

4. In your Wix editor, click the "+" button to add a new element.

5. Select "Embed Code" > "Popular Embeds" > "Embed HTML":

6. Paste your Kit form’s HTML into the box and click "Apply".

Tips: 

• Forms don’t show in Editor view, preview the live site in Wix
• Use “Inline Bars” in Kit (ConvertKit) if you want always-visible forms
• Set up automation inside Kit (ConvertKit) after someone subscribes

FAQs

I do hope this guide helps! For assurance, you may also look into this video (not mine).