Hi @bari86, the website you have shared does not specify the full details of the required HTTP Request. Instead it seems to simply set a bunch of variables in the PHP programming language.
Perhaps you can try sending them in a simple POST Request to start with? Something like this (using your own values of course):
This might still lead to an error, but this error is hopefully a bit more verbose than the documentation
I remember using other Automator software to connect WHMCS and to connect you need to insert more details, and write an line of code with a secret text that you will choose