Hi,
What kind of restful API you need to call and I will give you a code?
Usually in the task there is a thing called "Trigger" where we can code in PHP and if we are going to perform restfulAPI call, best we code it using PHP Curl method (that is what I have done before) and I saw some people now using javascript AJAX to do it.
If you want further learning refer to this documentation
https://wiki.processmaker.com/index.php ... _Endpoints