There isn’t a way to access the Webflow CMS API directly from client-side script, so you need a middle-tier.
There are a lot of popular 3rd party automation services as well like Make.com that can perform CMS updates.
Thank you for your answer.
Then how does the registration work?
I’m not sure what you mean by registration here.
You can’t do that directly from the web browser, Webflow blocks this. If you try you’ll get something called a CORS error showing up in your web browser console.
You have to have your web browser JS call some backend that does what you need with the CMS (so it can securely store your API key / password) and then returns that (as a JSON blob) back to your web browser’s custom code for display.
You can very easily do this with Webhook Endpoints.
That linked tutorial walks you through the basic concepts and shows you how to do it with Airtable.
Will show you how to submit data to Make (Integromat) where you can then update the Webflow CMS, and it returns a JSON blob that some custom code can now display.