You can't use AJAX, because of the javascript, to post to another domain. The issue is "cross domain scripting" and browsers prevent it since it can lead to security issues and be used maliciously.
Is the domain one you control? Is it on the same server? Using your own server page to do the work is an alternative. For example if the page is php then using curl can be a way to have your page work with the page on the other server.
If you want details please let us know what language you can use.
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