How to set a cookie for another domain

Question

Say I have a website called a.com, and when a specific page of this site is loaded, say page link, I like to set a cookie for another site called b.com

Answer 1

Send a POST request from A. Post requests are on the serverside only and can't be accessed by the client.

You can send a POST request from a.com to b.com using CURL (recommended, serverside) or a hidden method="POST" form (clientside). If you go for the latter, you might want to obfuscate your JavaScript so that the user won't be able to understand the algorithm and interfere with it.

Make a gateway on b.com to set cookies:

If you want to bring security a step further, implement a function on both sides (a.com and b.com) to encrypt (on a.com) and decrypt (on b.com) data using a cryptographic cypher.

If you're trying to do something that must be absolutely secure (e.g. transfer a login session) try oAuth or take some inspiration from https://api.cloudianos.com/docs#v2/auth