Next Auth authorize request header is of wrong type

I am trying to make Credentials based authentication to my project. However, when I sign in from the default sign in page the request is of type "application/x-www-form-urlencoded" even though in my [...nextauth].ts file specify the header to be of type "application/json". What am I doing wrong or how can I change the request header to be of type "application/json" correctly?

Here is my [...nextauth].ts file:

import { NextAuthOptions } from "next-auth";import NextAuth from "next-auth/next";import CredentialsProvider from "next-auth/providers/credentials";export const authOptions: NextAuthOptions = {        providers: [            CredentialsProvider({                name: "Credentials",                credentials: {                    email: { label: "Email", type: "email", placeholder: "example@company.com" },                    password: { label: "Password", type: "password" }                },                async authorize(credentials) {                    const res = await fetch("http://localhost:3000/api/user/login", {                        method: "POST",                        headers: {"Content-Type": "application/json",                        },                        body: JSON.stringify({                            email: credentials?.email,                            password: credentials?.password,                        }),                    })                    const user = await res.json()                    if (user) {                        return user                    } else {                        return null                    }                }            })        ],        session: {            strategy: "jwt",        }}export default NextAuth(authOptions)

I've tried to identify the problem from the tutorial I was watching and searching online for similar problems but couldn't find anyone having similar problem. Seems like the aforementioned code should work but for some reason for me the content-type header is still wrong.