If I want to be able to send a POST request to a web server hosted on the localhost, do I have to do anything in the manifest? I have added https://localhost in the list of externally_connectable URLs, but the event doesn’t go through
First of all, you need to check if you are using a self-signed certificate.
If so, you have to accept it first: double-clicked on the request in the Network pane, and OW prompted you whether you wanted to accept the certificate.
Other option will be to use the overwolf.web.sendHttpRequest() function.
Adding a small addition: Using the fetch API in no-cors mode will not help here, as it makes the request opaque, which means you won’t be able to see the content.