Express Gateway - Cannot change Content-Type value in HTTP request Headers

Question

I'm doing the Microsoft OAuth 2 authentication via Express Gateway, I've hardcoded the body to pass client_id and client_secret as per required. But when I call the Express Gateway API from Postman, if I don't pass any Body value, the Content-Type of Headers will be set as "content-type":"application/json" as default, which leads to failed authetication.

I've tried to change it to "content-type":"application/x-www-form-urlencoded" but it's not working, the log still show the default value. Here is the code:

Can someone please help me with this.