Set up a cross-domain AJAX

There are many landing pages in pure HTML + CSS + JS. There is a CRM that accepts all orders. All landing pages and CRMs are on different domains. When sending data about the application for the CRM domain from the landing domain, there is a ban on the browser side. You need to configure cross-domain AJAX requests from landing pages with any domain to the CRM domain. At the same time, when changing the CRM domain, the ability to send cross-domain requests should also be preserved.