Login to your Wix Account and visit your Dashboard. There you can see the your Website Name. Just click the Site Actions Dropdown and then click Edit Site. See screenshot below.
Now Click on the + Add icon from the left menu and choose Embed => Custom Embeds => Embed a Widget. See screenshot below
That's it. Now you can Save & Publish your page. Then open the page you've added the form in the browser. You might need to clear the cache if you don't see the changes instantly.