This article is about the Proxi admin app at app.proxi.co.
Each of these takes the Proxi embed code, with one thing to watch on each. Copy the code from Proxi first (see Adding Proxi to Your Website: Which Code to Use).
Shopify
Go to Online Store → Themes → Customize, open the page, click Add section, and choose Custom Liquid. Paste the code there. Don't paste it into a page's regular text editor, which removes code when it saves.
Duda
Click Add Widget, search for HTML, add it, and paste. Needs the Team plan or above. The widget's editor previews in a locked-down frame and may show a message about "cookie" or "sandboxed"; publish and check the live page.
Carrd
Add an Embed element and paste. Needs a Pro Standard plan or above. Carrd shows nothing in the editor for embeds, so publish to see it.