Getting 400 Bad Request Error When Making POST Request to obtain JWT Token Using HttpService in NestJS

I’m developing a NestJS application where I need to obtain a JWT token by making a POST request. I’m utilizing the HttpService from the @nestjs/common library. However, I consistently encounter a 400 Bad Request error.

Here’s the code using HttpService to get JWT token:

import {
  HttpService,
  InternalServerErrorException,
} from "@nestjs/common";


export class JwtService {
 constructor(private httpService: HttpService, private cache: CacheService) {}
private getAuthToken(): Observable<OAuth> {
    const url = 'endpoint_url';
    const body = JSON.stringify({
      "client_id": 'XXXXXXXXXXXXXXX',
      "client_secret": 'XXXXXXXXXXXXX',
      "grant_type": "client_credentials",
    });

    const headers = { "Content-Type": "application/json" };
    return this.callAPI(url, body, headers);
  }

private callAPI(
    url: string,
    body,
    headers: { [key: string]: string }
  ): Observable<any> {
        return this.httpService.post(url, body, headers).pipe(
          map((result: AxiosResponse<any>) => result.data),
          tap((result: any) => {
            const { access_token } = result; // eslint-disable-line
            this.cache
              .set("authToken", access_token, {
                ttl: env.SECONDS_TO_EXPIRE,
              })
              .subscribe();
          }),
          catchError((err: any) => {
            return throwError(
              new InternalServerErrorException(err.message, "Auth Service")
            );
          })
        );
      })
  }
}
}

Here’s what I’ve already tried:

1. Other APIs: I tested the same code with other APIs, and it worked flawlessly. The issue seems specific to the JWT token API.
2. Postman Success: When I manually send the request using Postman (same URL, body, and headers), I successfully receive the token.
3. Body Format: I attempted using a plain object for the request body instead of a JSON object, but the error persists.
4. Fetch API: I also experimented with the fetch() API, and it returned the expected response (code snippet below).

Sample code using fetch api:

const req = {
      method: "POST",
      headers: {
        "Content-Type": "application/json",
      },
      body: JSON.stringify({
        client_id: XXXXXXXXXX,
        client_secret: XXXXXXXXXXX,
        grant_type: XXXXXXXXXXX,
      }),
    };
    fetch('jwt_token_endpoint', req)
      .then((response) => response.json())
      .then((data) => {
        this.logger.log(`Success retrieving jwt token`);
        console.log(data);
        this.jwtSubject.next(`${data.token}`);
      })
      .catch((error) => {
        this.logger.error(`Error retrieving jwt token: ${error}`);
      });

I am looking for an assistance to resolve the error and get the token successfully.