Apache (.htaccess)
ErrorDocument 404 /404.html

Nginx
error_page 404 /404.html;

Vercel (vercel.json)
{ "routes": [{ "handle": "filesystem" }, { "src": "/(.*)", "dest": "/404.html" }] }

Next.js
Create pages/404.js or app/not-found.js

Then paste the embed code into that 404 page.

data-token — Your widget token (required)
data-mode — "page" (full takeover) or "chat" (floating bubble)
data-position — "bottom-right" or "bottom-left" (chat mode)
data-theme — Force a theme or "auto" for random