I have a declared variable at the JavaScript library, i want to replace it in a POST variable.
how can I call it and use it in a javaScript action and replace it in a POST variable?
You need to store your JS variable into a NeoLoad variable using the "context.variableManager.setValue method.
When you drag and drop a new JS action in your script there's a skeleton with some examples on how to call and store into a NeoLoad variable.